I have already tried sgmixer, but it doesn't seem to have any effect whatsoever (except if I am missing something).It has always worked here directly from the install, including booting base norestore, and across a variety of hardware.
I had also looked at link of aumixer, but it seems to be available only as source.Packages for deb and rpm are available from other distros, might be worth investigating converting to tcz:
Mediocre minds thinking alike, I have already tried aumixer from debian repos in the meantime following that method, and several other GUI or curses mixers which I found there supposed to work with OSS as well.
It appears that they all default to use /dev/mixer which seems not to work, and I am clueless what device to specify.
The only exception is ossxmix, where the debian version worked out of the box, no noticable difference to ossxmix.tcz
Also notable that mp3blaster works perfectly well with OSS, but unlike with alsa the builtin mixer does not work, this on same box.
Besides from ossxmix, the only app I have so far found to work for adjustment of sound with OSS is flit, which of course is limited to output volume and muting, still comes very handy though.