So I was playing a bit of FFII earlier, and I was going through the Coliseum part. Up comes a random encounter and it's something like Captain, Sergeant x2, Wizard x2 , Sorcerer. My first reaction is "Oh crap," because it was kind of strong for me at that point, and it gave me a Game Over as I was expecting.
My second reaction though was, "Wait, Wizards?!". Wizards of course are the only way to get more Osmose books after the one in the Mysidian Cave. And there were only three places I knew about that they showed up: Castle Fynn upper floors, the Cyclone, and Castle Palamecia. ALL one-time dungeons, which makes farming eneies in any of those areas annoying (especially since Cyclone is the only one that allows teleports out normally). So I loaded the end game save I had, when over to the Coliseum and looked around for another encounter with Wizards. Eventually, I got one. This was a pretty good discovery, because it means that Osmose books can be farmed without having to rely on one-time only areas.
'Course there's probably an encounter list for the game out there I'm unaware of that has this information. Tracking down some of the more detailed information for this game gets tricky sometimes.
Yeah, I had the fortune of running into the Captain-Wizard group on my first playthrough and was viciously bombarded with Flare spells. And it certainly didn't help it happened after Gordon ran off with Hilda.
This site has enemy encounter info: http://guides.gamercorner.net/ffii/
The data is for the NES version, but it's probably the same in the Origins version. Considering Wizards rarely appear in the Coliseum and Osmose is also a rare drop, getting an Osmose will likely take a long time.
Because Osmose is basically infinite MP. FFII has a very limited inventory, so you can't stockpile lots of curing items. Potions, Hi-Potions and Ethers don't recover much either. So it's more effective to just cast Cure, Esuna, and Life/Raise than stockpile potions, status cures, and Phoenix Downs. Also, magic remains very useful into the endgame, where it's good for buffing the party and casting instant death spells on the enemies. Ethers only restore around 40 MP, where Osmose can be continually used to get more MP back from random encounters. Osmose is consistanly considered one of the must-have spells in the game.
However, the book isn't sold anywhere, and there's only 1 chest that has the book. The only other way to get it is from the Wizards.
I beat the game as a solo using only magic, and osmose was probably the only reason it was possible. Or at least it made it pretty easy. If you grind and stock up on tons of ethers I guess you could do it without.