FreeBSD's OSS is not ALSA.
ALSA refused to do mixing in software for some time. Maybe it still doesn't do it, I don't know. So if you had a hardware sound card that would do mixing, you could play multiple channels with ALSA, but if you didn't, you were SOL.
Alsa has had dmix for quite some time. The thing was that until something like a decade ago it had to be manually enabled via a .asoundrc edit. These days Alsa will enable it automatically on any device known to not have a hardware mixer.
Maybe so, but I did try to fix it without resorting to a hardware sound card and couldn't at the time. IIRC the syntax of the ALSA configuration files was particularly arcane.
Not sure why that deserves downvoting.
Now, it's just anecdotic evidence of course but basic audio mixing is not exactly rocket science, you shouldn't need something as convoluted as PulseAudio to get it right. I mean, what the hell: https://gavv.github.io/blog/pulseaudio-under-the-hood/diagra...
Some power users might need these additional features, I don't. I wouldn't mind having all of that if the basics just worked reliably but they don't.
These days disabling PA is my go-to first step while troubleshooting audio issues on linux and more often than not I don't have to go to step #2.