I'll skip the formalities since with 2 OBSD machines and 1 FBSD you *surely* have a clue about what it is you're doing.
NFS is just like the handbook says so far as I know. Edit /etc/exports, HUP mountd and mount the export on the client. Linux's NFS implimentation doesn't always play well with others (big surprise) but I've never had problems from BSD to BSD (though the *exact* error would help a lot, obviously).
As for why that would hose up the DNS on one machine and the routing table on another ... I can't think of any way that it could happen, but if something comes to me, I'll post back.
Oh yeah, and your fstab should look like this:
fileserver:/OBSD/usr/src /usr/src nfs 0 0
You don't want the filesystem dumped or fscked at boot time...