my general rule is that it's definitely OSR if it's compatible with every edition of D&D. So anything that does that is definitely allowed.

But it's not like anyone's saying that the Black Hack isn't OSR, and I'd accept submissions for Black Hack content.  

While trying to figure out what BrightHammer was, so that I could answer this question, I found out that the person who wrote BrightHammer has me pre-emptively blocked on Twitter, so I'm not gonna accept submissions on that. 

Basically, if it's not generically compatible with a bunch of stuff, I guess directly ask me and I'll look it up. I'm generally going to err on the side of yes, though, on the basic assumption that a) you and everyone else is acting in basically good faith and b) not that much hinges on me getting the exact definition of "OSR" wrong. 

For what it's worth, my working guideline of OSR is "A character or monster made in an OSR game should be able to have a fight with a character or monster made in an old-school D&D game with minimal issues." In particular, both should use d20 to attack and track damage in hit points. Also an OSR game should use the same six ability scores as D&D, possibly renamed.

ETA: I don't mean that this should be everybody's definition. It's just the one I have in my head.

I will be introducing a Swords & Sorcery supplement for Swords & Six-Siders which is OSR compatible with some modifications. Would that work for this scenario?

is there a complete list of what has so far been determined as definitely acceptable system-wise?

This chart might be a handy guide for the starting point of OSR. Not saying this is definitive but it might be helpful for someone to pick an edition they like and start from there.

edit: for the original picture