This is normally accomplished by adding a route filter in OSPF as follows:
/routing filter
add chain=ospf-in set-pref-src=[/routing ospf instance get default router-id]
However, this “broke” or changed somewhere between 6.3 and 6.30.4 (I didn’t test in which exact version it changed). Apparently it’s “fixed” in 6.33rc according to this thread: http://forum.mikrotik.com/t/when-fastpath-works-pref-src-not-work/91356/1
Joe