Hmm, I haven't much used OSS. Too bad you've already installed while using the alsa cheatcode before installing generally gives a better sound system.
I didn't see the sound module so you might want to try with modprobe sound and see if you can tinker with any mixers from ll /usr/bin|grep mix or kmix after that. No need to try alsamixer or aumix with OSS. Also if you use KDE have a look in Control Center -> Sound & Multimedia -> Sound System, and check "enable the sound system" to see if kmix would work with arts.

EDIT: Come to think of it, you can get a potentiometer for less than 1 €/$ or from any old scrapped piece of electronics. A bit of soldering and you have a volume control. (sorry for potentiometer being in swedish, I hope it's the same in english)