- OK-MECH Domain Overview
- Domain Weight Breakdown
- Domain 1: Electrical Knowledge and Controls
- Domain 2: Natural Gas
- Domain 3: Air Distribution Systems
- Domain 4: Refrigeration and Air Conditioning Systems
- Domain 5: Heating Systems
- Domain 6: Safety
- Exam Format and Registration Mechanics
- Building a Study Schedule Around the Domains
- Who This Domain Knowledge Serves on the Job
- FAQ
- The OK-MECH trade exam has 90 scored questions across six domains, weighted 15-12-25-20-15-3.
- Air Distribution Systems carries the most weight at 25 of 90 questions - study it first.
- Safety is only 3 questions but still requires attention since every point counts toward 70%.
- You get 240 minutes total, plus extra unscored time PSI may add for up to 10 experimental items.
OK-MECH Domain Overview
The Oklahoma Mechanical Trade License Exam for HVACR Contractor Unlimited (OK-MECH) is administered by PSI Services LLC on behalf of the Oklahoma Construction Industries Board (CIB). It is a 90-question, computer-based trade examination, separate from the 50-question Business and Law portion, and it is built around six defined content areas that map directly to what an Unlimited-tier HVACR contractor actually does in the field: work on equipment rated above 25 tons and 500,000 Btu/h.
Understanding these six domains isn't optional trivia - it's the single most efficient way to allocate your study hours. If you've already read our OK-MECH Study Guide 2026, this article goes deeper into exactly what each domain covers and how much of the 90-question exam it represents.
Domain Weight Breakdown
The current PSI bulletin (effective August 31, 2026) confirms the six official subject allocations for the trade portion of OK-MECH. Here is exactly how the 90 scored items are distributed:
| Domain | Approx. Question Count | Share of Exam |
|---|---|---|
| 1. Electrical Knowledge and Controls | 15 | ~17% |
| 2. Natural Gas | 12 | ~13% |
| 3. Air Distribution Systems | 25 | ~28% |
| 4. Refrigeration and Air Conditioning Systems | 20 | ~22% |
| 5. Heating Systems | 15 | ~17% |
| 6. Safety | 3 | ~3% |
Remember, PSI may also insert up to 10 unscored experimental questions into any candidate's exam. You won't know which items are experimental, and answering them doesn't add to your 240-minute clock, but it does mean the exam may feel slightly longer than 90 questions on screen. For a deeper look at how this affects overall difficulty, see How Hard Is the OK-MECH Exam? Complete Difficulty Guide 2026.
Domain 1: Electrical Knowledge and Controls
Electrical Knowledge and Controls (15 questions)
This domain tests your ability to read, troubleshoot, and safely apply electrical concepts as they relate to HVACR equipment controls - not general residential wiring. Expect questions on control circuits, motor protection, and component sequencing within larger mechanical systems.
- Control circuit logic for HVACR equipment (relays, contactors, transformers)
- Motor starting and protection devices used on Unlimited-tier equipment
- Basic electrical troubleshooting sequences tied to system controls
- Interpreting wiring and control diagrams for commercial-scale units
Domain 2: Natural Gas
Natural Gas (12 questions)
Natural Gas questions focus on fuel-gas piping, venting, and combustion safety as applied to gas-fired heating and process equipment. This is one area where your open-book fuel-gas code reference becomes essential during the exam.
- Gas piping sizing and material requirements
- Venting and combustion air requirements for gas appliances
- Code-based clearance and installation rules from the fuel-gas code
- Gas train components and safety shutoff logic
Key Takeaway
Practice locating specific tables and clauses in your fuel-gas code reference before test day - Domain 2 rewards speed at navigating the open-book material, not memorization of every clause.
Domain 3: Air Distribution Systems
Air Distribution Systems (25 questions - the largest domain)
With 25 of 90 questions, Air Distribution Systems is the single highest-value domain on the OK-MECH trade exam. It covers ductwork design, airflow calculations, and equipment sizing for commercial-scale distribution systems - exactly the kind of work Unlimited contractors sign off on.
- Duct sizing and airflow calculations using an ACCA ductulator
- Static pressure concepts and system balancing basics
- Register, grille, and diffuser selection for airflow requirements
- Ductwork material and installation code requirements
Domain 4: Refrigeration and Air Conditioning Systems
Refrigeration and Air Conditioning Systems (20 questions)
This domain is the technical core of HVACR work - refrigeration cycle theory, component function, and troubleshooting for cooling equipment sized well above residential scale, consistent with the Unlimited tonnage threshold.
- Refrigeration cycle stages and pressure-temperature relationships
- Compressor, condenser, evaporator, and metering device function
- Refrigerant handling concepts drawn from your refrigeration textbook reference
- Diagnosing common system faults on larger commercial units
Domain 5: Heating Systems
Heating Systems (15 questions)
Heating Systems covers combustion-based and electric heating equipment installation, controls, and code compliance, tying together concepts from both the Natural Gas domain and the mechanical code reference.
- Furnace and boiler component function and sequence of operation
- Combustion safety controls and limit switches
- Heat exchanger and venting requirements for heating equipment
- Code-based clearance and installation standards for heating appliances
Domain 6: Safety
Safety (3 questions)
Safety is the smallest domain by question count, but it draws on your OSHA reference material and covers job-site hazard awareness relevant to mechanical contracting work.
- General job-site hazard recognition
- OSHA-referenced safety practices for mechanical trades
- Safe handling procedures around pressurized and electrical equipment
Because Safety is only 3 of 90 questions, don't over-invest study time here relative to the higher-weight domains - but don't skip it either, since every correct answer helps you clear the 70% passing score explained in full in OK-MECH Passing Score 2026: Exactly What You Need to Pass.
Exam Format and Registration Mechanics
Knowing the domains is only half the picture - you also need to understand exactly how the exam is delivered and how registration and fees work under the CIB and PSI system.
- Format: 90 scored questions, computer-based, 240 minutes, plus up to 10 unscored experimental items that don't count against your time.
- Passing score: 70% on the trade exam; the separate Business and Law exam (50 questions, 120 minutes) also requires 70%.
- Trade exam fee: $92 per attempt; the Business and Law exam is a separate $92 fee.
- Initial licensing cost: $330 total to the CIB - a $30 application fee plus a $300 license fee.
- Retake waiting periods: 30 days after a first failed attempt, and 90 days after any subsequent failure.
- Three-year window: Both required exam portions must be passed within three years of eligibility approval.
- Open-book references allowed: specified mechanical and fuel-gas codes, a refrigeration textbook, an OSHA reference, ACCA Manual N, Mathematics for Plumbers and Pipefitters, and an ACCA ductulator. Pre-highlighting and permanent tabs are permitted; handwritten notes are not.
- Calculator: a PSI-compliant non-programmable calculator is allowed.
For the full cost picture - including bond, insurance, and renewal costs beyond the exam fees - see OK-MECH Certification Cost 2026: Complete Pricing Breakdown. Active contractor licensing also requires a $5,000 surety bond and $50,000 commercial general liability insurance, which is separate from the exam fees themselves. If you're not yet sure whether you meet the underlying experience and age requirements, review OK-MECH Requirements 2026: Eligibility, Prerequisites & How to Qualify before you register through PSI.
Building a Study Schedule Around the Domains
Rather than studying generically, map your prep time directly to the domain weights above. A simple four-week structure that respects the 25-question Air Distribution domain and the 20-question Refrigeration domain looks like this:
Air Distribution Systems + Refrigeration and AC Systems
- Work ductulator and airflow calculation problems daily
- Review refrigeration cycle diagrams and component functions
Electrical Knowledge and Controls + Heating Systems
- Practice reading control diagrams and sequence-of-operation questions
- Study furnace/boiler combustion safety controls
Natural Gas + Safety
- Practice locating fuel-gas code tables under time pressure
- Quick-review OSHA safety reference material
Full-Length Practice + Reference Tabbing
- Take timed full 90-question practice sets on our OK-MECH practice test platform
- Finalize permanent tabs in your code books and ductulator
This weighted approach is far more efficient than studying all six domains equally, since Air Distribution Systems and Refrigeration and Air Conditioning Systems together make up half the scored questions. For additional pacing strategies, see OK-MECH Study Guide 2026: How to Pass on Your First Attempt.
Who This Domain Knowledge Serves on the Job
The six OK-MECH domains aren't arbitrary - they reflect the real scope of an HVACR Contractor Unlimited license in Oklahoma. Passing candidates go on to run or work for mechanical contracting firms that install and service equipment above 25 tons and 500,000 Btu/h: commercial buildings, industrial facilities, large multi-family properties, and institutional clients. Employers and clients rely on the license as proof that a contractor can handle electrical controls, gas piping, ductwork, refrigeration, heating equipment, and job-site safety at that scale - not just residential-sized units.
If you're evaluating whether pursuing this license fits your career plans, our guides on OK-MECH Salary Guide 2026: Complete Earnings Analysis and Is the OK-MECH Certification Worth It? Complete ROI Analysis 2026 walk through the broader picture beyond the exam itself. You can also browse OK-MECH Jobs for a sense of the roles this license opens up.
Once you're confident in the domain content, run a full simulated exam on our practice test platform to see how your timing and accuracy hold up under the same 240-minute structure you'll face on test day.
Frequently Asked Questions
Air Distribution Systems is the largest domain, with 25 of the 90 scored trade exam questions - more than a quarter of the exam.
The trade exam has 90 scored computer-based questions with a 240-minute time limit, plus up to 10 additional unscored experimental questions that don't count against your time.
No. The six domains covered in this guide belong to the trade exam only. Business and Law is a separate 50-question, 120-minute exam with its own $92 fee and 70% passing requirement.
Pre-highlighting and permanent tabs in your approved reference materials are allowed, but handwritten notes are prohibited. A PSI-compliant non-programmable calculator is also permitted.
You must wait 30 days after a first failed attempt and 90 days after any subsequent failure before retesting, and both required exam portions must be passed within three years of your eligibility approval.