While we're talking about third party applications and header file
   compatibility, does anybody know why SunOS uses O_SYNC as its flag to
   tell the kernel to do synchronous writes, while NetBSD, BSD/OS and
   probably 4.4BSD use O_FSYNC?

   [...]

   Is O_FSYNC in the POSIX spec or something?

I suspect this is another bit of history where Berkeley did one thing,
and AT&T/USL later did something substantially similar but not *quite*
the same.

The current revision of POSIX.1 does not specify the flag by either
name.