AFAIU, you don't need to use libc on OpenBSD, however only one contiguous region of code is permitted to invoke syscalls, and so you can't mix-and-match ad hoc

AFAIU, you don't need to use libc on OpenBSD, however only one contiguous region of code is permitted to invoke syscalls, and so you can't mix-and-match ad hoc libraries. See https://man.openbsd.org/msyscall https://man.openbsd.org/msyscall, which implements one-time registration of a contiguous range of pages permitted to invoke syscalls.