[1] Its counterpart, a capital expenditure (capex), is the cost of developing or providing non-consumable parts for the product or system.
For example, the purchase of a photocopier involves capex, and the annual paper, toner, power and maintenance costs represents opex.
[2] For larger systems like businesses, opex may also include the cost of workers and facility expenses such as rent and utilities.
[4] In TOC, operating expense is limited to costs that vary strictly with the quantity produced, like raw materials and purchased components.
Everything else is a fixed cost, including labour (unless there is a regular and significant chance that workers will not work a full-time week when they report on their first day).