Comparing how they mesh with my other two favorite transhumanist settings, Transhuman Space and Eclipse Phase: THS meshes quite well with OA although it requires fiddling with the latter's timeline. But it doesn't work too well with Eclipse Phase for multiple reasons, one of them being that (non-destructive) uploading is exclusively S:1 transapientech.

As for SA, it meshes very well with THS, even better it's 100% compatible without changes to either. For EP/SA, an important consideration is that they both have the same license. Their uploading tech translates between brain and digital easier than SA says you can, which has this to say in the Nonexistent Tech section:

"Brain Duplication by “downloading” someone’s
personality isn’t allowed. You can make an
imperfect copy using a mesh, which results in a
Persona Lens or you can make a perfect copy by
feeding the person into a replicator. One could
also make a very, very good copy by using a mesh
over the course of about 10-15 years."

This seems to fit Eclipse Phase to a T, however I actually feel EP does a good job of justifying their "no death" game mechanic with their descriptions of cortical stacks. The author of SA either doesn't agree or just hasn't gotten around to reading EP yet. Either way for a crossover preserving that aspect of EP would be important, so that statement can be disregarded.

Another thing that would conflict is obviously the psionics. But you could definitely change it so they're unknowingly akin to Technomagi, in fact I'd say it's heavily implied to be how it works if not outright stated.

The Prometheans in EP behind Firewall are obviously the Transcendentals.

Perhaps make EP the only civilization of SA that stayed within the solar system, and their cortical stack technology a Civilization Benefit. Only problem is it might be a bit too powerful compared to other civilization benefits, and so you might make it instead a thing all civilizations use. But I would think that would change the setting quite a bit, not sure though.

The whole Orion's Arm setting is heavily based off of Buddhism, while Sufficiently Advanced is not. This one reason, another being the different tech, that a direct crossover between the two would be difficult. As I suggested above though, you could simply make it Orion's Arm-like by having the building of singularity-level godbrains be a future event in SA. This would be somewhat fraught with tension considering the Aia, but OA has a relatively similar concept with ahuman ais.

However the early ahuman ais were much less capable than the Aia, indeed they were basically autistic, and the Aia are much more complex than even the archailects are said to be (not more intelligent though) and quite a bit more chaotic than any ahuman ais, so the simliarities between ahuman ais and the Aia is only superficial. One big important difference is that no pro-human equivalents of Aia stepped forward like in OA, but it's still a possible future. I would imagine if pro-human archailects got made in SA, they'd be more similar to OA's conception, i.e. much more ordered than the chaotic Darwinian nature of the Aia.

I find it difficult to imagine what shape SA would take if it had pro-human archailects like OA. Certainly it wouldn't look anything like Orion's Arm, except perhaps in broad strokes. It would probably still be a setting you could turn into a game better than Orion's Arm. :P

Secondly and relatedly, where OA has achailects and lower transapients ruling most of the setting, those sorts of giant computers/ascended beings do not appear to exist in SA, outside of the Aia and Transcendentals. This is probably just a difference in social histories, looking SA it seems reasonable to suppose it didn't happen like in OA because of bad luck. My guess the Transcendentals Desired Future looks similar to OA in this respect, lots of newly ascended beings looking out for a revitalized (trans)humanity.

If they did exist, all technology that is implied to be in the future would've been developed a long time ago.

As far as Capabilities go:

Biotech: OA does not have immortality without treatments, and looking at the page of Superior genemods, they seem to be a solid Biotech 3. However, on the civilization as opposed to personal level they do have quite impressive biotech, just maybe not to the degree as SA.

On another hand, they do have asomnia, which while not explicitly to not exist in SA, if it did would've probably been mentioned either in the Biotech section or in the Needs box (one of the needs requires sleep). Although it could just be uncommon ATM like in OA.

Chronotech: This does not seem to be a thing that exists in OA.

Cognitech: Modosophonts in SA seem to get much better performance from Cognitech before hitting the toposophic barrier, due to it seemingly not existing. On the other hand, no giant superitelligent computers like in OA. The Stored sorta count, but they're still not as smart as in OA. So overall seems mixed.

Metatech: Overall Metatech looks much more powerful in SA. Psychohistory for example is impractical in OA even for the ascended beings, while it's a standard and highly effective thing for modosophonts in SA.

Nanotech: I'm not really positive, but they seem about on par, maybe.

Stringtech: While OA does have achailect level stuff like basement universes, overall its level of Stringtech seems much lower. They don't have most 4-5 tech, they do have transmutation but it's much less efficient. SA also has remote wormholes while OA only has local.

Another setting difference is uplifts. Uplifts are ubiqitous and important in OA, but if they exist in SA they're not as common or important, as they would've been mentioned in the Mechanicans' Humanity CV otherwise. But as with asomnia above, I prefer to to headcanon SA as being the same as OA in terms of uplifting ubiquity and significance, as I really like the idea of uplifting.

I'm sure there's a bunch of other stuff, but OA is a pretty big worldbuilding project so I'm only passingly familar with it.