OpenMP versus std::thread/pthread/etc isn't this distinction. Both are thread-based parallelism; that OpenMP implementations do so cleverly via a threadpool approach is secondary. Any loop you can write using OpenMP you could dispatch manually via an explicit thread-based approach, and long-running parallel task you'd normally implement via std::async/std::future/std::thread could be dispatched via OpenMP. There are good conventions about what kinds of operations are better expressed via one or the other mechanism, but they are effectively equivalent in capability.
