This port provides a simple and easy to use
tool to provide fonts in OSD.
Just cd to the ports directory and type

make install-user

Note: This is only for the bitmap fonts! If you've compiled
mplayer with truetype support, this won't work.

Please take a look at docs in
/usr/local/share/doc/mplayer

If you encounter strange problems, please have a look at
http://www.rrr.de/~riggs/mplayer/faq_en.html
before throwing stones at me :-)