The O_DIRECT flag has been added in FreeBSD 4.4 (386 & Alpha) also. From
the release notes:
Kernel Changes
The O_DIRECT flag has been added to open(2) and fcntl(2). Specifying this
flag for open files will attempt to minimize the cache effects of reading
and writing.
See: http://www.freebsd.org/releases/4.4R/relnotes-i386.html