While I have no experience with OSS.
When I built mpd, I have also built support for OSS (see info-file). However, I've never tested - maybe someone else?
Probably the problem comes with the support for jack and pulseaudio. In the list of dependencies of jack and pulsaudio is also alsa. Now you have both sound systems (alsa and OSS). Just try the following :
/usr/local/etc/init.d/alsasound start
speaker-test
When it roars, alsa has won the race ...
Need necessarily OSS?
AFAIK, OSS is "deprecated" since kernel > 2.4 and alsa is backward compatible with the extension alsa-oss.tcz ...