PulseAudio has already been abandoned, the new thing is Pipewire.
The specific issue with OSS was that it was never part of Linux and then with OSS 4 they made it proprietary, which killed it off entirely as far as Linux users are concerned. (FreeBSD cloned it instead.)
Oh yeah. I should have noticed that because on the Debian machine I set up for my daughter, audio suddenly broke, and it started working again when I removed pipewire.
No joke, if you remove the new thing stuff magically starts working again.
I had to restart Pipewire just this morning when audio randomly stopped working. It's funny as this is the first time it happened then an hour later I'm reading in this thread about Pipewire acting funny on other people's computers.
in my experience, unlike pulseaudio, pipewire is way stabler and usually works fine with less involvement.
... except when you have accidentally ended up with half-pipewire, half-pulseaudio setup due to half-forgotten instructions that were no longer applicable.
I ended up solving a similar case for someone else, where ultimately different programs were fighting over control of the sound devices themselves.
I have to say, that pipewire under NixOS had been easier to deal with than both pulseaudio and ALSA on crappy internal sound devices (i.e. ones requiring dmix and the like). It's nearly as plug&play as ALSA on "proper" soundcard was (like when I got a Dell Precision to work with that somehow had a Creative Audigy with 256 channel sound, so no need for dmix at all...)
Open Sound System (OSS), rather than Open Source Software, if I'm reading you right. I only recall because I've been toying with sound software for decades, trying to get various synths and music composition applications working. Apologies if that didn't address your confusion.
Iirc there was a version in the kernel tree, then "upstream" if you could even call it that developed it further (OSS 4.0). I think there was a company behind it and a nonfree license. Obviously that was a no-go for inclusion into mainline Linux.
But alsa was/is also enormously complicated with a huge library, user mode plugins and whatnot. It reminds me of a common problem that people have where they think API and implementation are one and the same, and that you cannot swap out a different implementation keeping the simple API. People would say you need alsa because it does software mixing, but the lack of software mixing wasn't an OSS API problem, it was a problem with how it was implemented.
ALSA started replacing OSS on linux before software mixing became as critical as it ended up, because people still often had proper sound cards with multiple channel hw mixers.
The assumption that you could punt software mixing to hardware or deal with limitation of only one program accessing the audio at a time took a hard hit when Intel HDA pretty much decimated presence of such hardware on PCs (AC'97 was much less pervasive for various reasons)
The specific issue with OSS was that it was never part of Linux and then with OSS 4 they made it proprietary, which killed it off entirely as far as Linux users are concerned. (FreeBSD cloned it instead.)