Specifically what objectives are required to meet certification for Level D...which is only 'minor' criticality.
I can't seem to find anything, in the public, that points to what the objectives are for Level D. I'm hopeful that a general purpose OS would meet whatever the criteria are.
Disclaimer: you must consult with a certification professional or with your certification authorities for getting a formal decision.
Since your question regarding the objectives for Level D is too general, I’ll focus only on operating system: DO-178B does not specify any requirement on an operating system. Actually an OS is considered just as any other software. Nevertheless, DO-178B does specify an objective, relevant also for Level D development, which OS can come handy: partitioning of resources (space and time) and confirming its integrity.