Operating expenses are the costs of running the business that are not tied to any single job: rent, utilities, insurance, software, and admin. They are often called overhead.
The distinction that matters is job cost against overhead. Materials and the labour on a job are job costs and scale with the work. Overhead is there whether or not you sell anything this month.
Operating expense ratio
The share of revenue going to overhead. When it climbs while gross margin holds steady, the work is still priced correctly and the business has simply become more expensive to run.