MLGS is arguably the longest of greatswords, while OGS is perhaps the shortest. OGS R2 AoE is utterly useless outside of wake up attacks, where MLGS 2H R2 works just as well.
Unlike OGS, MLGS special attacks are quite usable in normal combat, especially if you learn to aim them unlocked.
Buff, or no, if you can't reach, the buff does nothing more than look cool.
MLGS has insane phantom reach and is the best roll punisher weapon in the game, next only to thrusting weapons.
Pure magic damage means big chip damage and outside of chipping, it's damage is comparable to a Claymore on a quality build.
Unlike OGS, MLGS can stunlock quite easily. Again, range thing.
MLGS R2s are made out of the swing itself and then the special attack, which means that if the target is close enough, he gets damage from both. OGS R2 doesn't have that, but on the other hand can't be parried.
OGS AoE, while looking all fire and all can be blocked (unbuffed), but MLGS does decent chip damage if you block it and can Dead Angle you if you aim the explosion behind the target.
As a matter of experience, I fought against buffed OGS quite a few times, especially when the DLC was still new. The range difference between OGS and MLGS is big enough that I can actually pull a R1 and evade before OGS can hit me and it's too short to punish my roll.