Possible issues with using MPlayer on OpenBSD:
all arches:
The GUI is deprecated so we stripped it from the build. You can use
x11/gnome-mplayer or x11/smplayer instead.
i386:
win32 codecs were disabled in OpenBSD's build, as the open-source codecs
are now of decent quality.
arm:
You may encounter slow video output. In this case you can disable
the X.Org screen rotation in /etc/X11/xorg.conf. Depending on the
video you are playing you may want to pass the -framedrop or the
-hardframedrop option to mplayer.
In order to improve vorbis performance on ARM, try using the tremor codec.
This can be achieved using '-ac tremor'.
See the package description for a list of FLAVORs and their meaning.