>      Having ported our project to NetBSD 1.3.3, we are obviously interested
> in porting to FreeBSD and OpenBSD as well.  A colleague claimed that BSD
> a.out objects should be usable as-is on all three OS's, but I've tried our
> exe's on OpenBSD 2.5 with no luck ("Bad system call").  And we are even
> statically linked with ALL libraries (libc, etc.), so it shouldn't be a lib
> compatibility problem.

My understanding is that OpenBSD and NetBSD have both added system calls
since the source trees diverged, and OpenBSD has never distinguished its
executables from NetBSD's so there's no easy way to do the emulation that
we do with FreeBSD, Linux, SVR4, et al.

For i386, anyway, you should be able to generate FreeBSD executables and
run them on all three.

Good luck!

-allen