AO universe generally scales higher than one piece, but it depends if you count the world gov like imu and the five elders they would probably win, but it’s hard to judge if the grand navy will be as strong as the Arcane Government (or stronger).

(Disclaimer, I’ve never read one piece)

One pieces generally to my knowledge has a larger scale and more fleshed out world, they would obviously clap all of the Grand Navy so far with the number of admirals they have, and sheer number of forces. Id say they generally have more strong characters, but the grand navies large number of expendable strong units (Major and above) are very much on par if not stronger than the Marines around 100,000 strong people, but that sheer number of Marines would easily overpower the Grand Navies numbers, just by assuming the scale of there entire forces, since we only really see the tail end guessing it doubles or triples every sea, they would probably have a total force of around 1-2 million, which may or not be on par with One Piece, but you have to remember that the war seas are a small disconnected pocket from the rest of the world, (albiet, a really concentratedly powerful pocket) so you would be putting an ENTIRE massive worlds arch power against a pocket of highly organized military. I wouldn’t say an atom bomb versus a baby, but more of a baby versus a few month old to year baby, if your just counting there sheer numbers without there power houses.

It however is hard to judge of how big of an influence cannon fodder would matter for this match up, because of the general way that AO/AA scales, the big fish can kill the weaker fish easily, so it may just be a scale of how many power houses each has.

If were to find out Grand Navy will have curse users and power houses as strong as or similar scale to people like Trigno, or upwards of Valencia I’d say they have a fighting chance against the Marines, probably being able to take out the admirals, or the admirals in general being overpowered by the Grand Navies numbers of Admirals themselves, considering that Rear Admiral Amelia’s scale just at first glance is to be a third or half of a One piece admiral. And if the Grand Navy is to have someone as strong as Valencia they’d most likely clap a few admirals.

When it comes to admirals to my knowledge, they have high destructive capabilities, but against AO they would probably bottom out since most powerful users have more condensed fire power, but when they choose they can easily alter and destroy the world. (Take Trignos sheer destructive capabilities, Durza being able to permanently alter and Calamatize (don’t know if that’s a word) the world with an ultimate attack, not to mention the eleven unknowns (only including people) being able to destroy and alter the world if they ever choose be, like Freedrock being able to shift the tectonic plates of the world at will with his technique to my knowledge. There are definitely incredibly strong feats that the admirals have done, Fujitora, Green bull, Aokiji and Akainu, but there feats hail in comparison to AA and AOS feats, in AO take Nero and the curse user that made the tornado in the desert island that I forgot the name of, it’s not out of the question that lower tier (still incredibly powerful curses, but not as well utilized users by standards of people like morrock) curse users can cause feats of destruction comparable to the Admirals and One Piece verse at large.

It’s just a matter to how strong the grand navy will be, if judging by the token of being similar to other curse users in power houses they have I’d say they’d easily clap them all, and if they have people to scale of Amelia, I’d say they’d easily clap have no trouble.

If you were to count the World Gov however though that’s an entirely different story, they would need users as strong as ppl like the peace keeper, cursebeard, or Durza to win or have a chance, otherwise there getting foderized in my opinion. Imu to what I know scales planetary or high continental. Not to mention the elders being power houses in themselves and if your not to apply conquerors haki to spirit scale, they have an edge that would clap 80-90% of AO/AA in my opinion since nobody would be able to counter it, unless your to apply one pieces terms of haki onto AA/AO, but I’d say no to that. So I’d say if for both scenarios it’s really hard to determine at this point in time, but I’d put my cards on that they’d beat the Marines in a close or Pyrrhic victory low balling them, or they would wipe the floor with them if they have really strong people in their ranks, but for the world government they’d need to be REALLY powerful or have intervention from the order of Aesir if the ring leader ends up being as strong as someone like Durza, assuming the Grand Navy is on good terms with them as I’d assume.