To gain Tidas’ overdrives, just keep using them over and over. After a while you will learn the next one up. There are a total of 4.
Yuna’s Overdrive - Master Summon
Summons any Aeon with filled overdrive gage. After the Aeon uses its overdrive once, its overdrive gage returns to the previous fullness.
Gaining Wakka’s Overdrives
You can win new overdrive roulette types in the Blitzball Ranking and Tournament modes. There are 4 additional Overdrives to get this way.
Lulu’s Overdrive
Simply rotate the right analogue stick clockwise as fast as you can. The number will go up the more you rotate the stick. I’ve gotten her to cast 8 times this way. The more you level Lulu, the easier it is to cast multiple times and the number raises.
Gaining Auron’s Overdrives
For Auron, you need to find different Jeckt Spheres laying about the world. Every time you view one of these spheres you learn more about Basaraka, Jeckt and Auron’s journey 10 years ago. Some of these will give Auron a new Overdrive. You need to find all 10 to gain Auron’s final overdrive. There are a total of 4.
Jeckt Sphere locations:
1) You get automatically after the Jeckt sphere event
2) Beisaid Village, to the right of the temple entrance
3) The ship that took you to Luca, Steering Room
4) Luka Stadium, B1 A, in front of the Ooraka Control Room
5) Mihen Road, Old Road South part, at the end of a dead end road
6) Mushroom Rock Road, cliff, around a save sphere
7) Shoopuf riding dock, South Shore
8) Thunder Plains, South part, around mid-point between the entrance and the Travel Company, near a thunder tower to the right
9) Makalania Forest, South part, around the save sphere right after entering from the Thunder Plains
10) Gagazetto Mountain, on the road up the mountain (this one you can get before acquiring the airship)
Kimari’s Overdrives
Kimari has an ability called Dragon Sword. Whenever you use this ability you drain HP and MP from the enemy. If the enemy has a blue magic skill that Kimari can learn, he will gain it automatically. His overdrive meter will also fill all the way up, allowing you to test it out. Just try out his Dragon Sword on all the enemies you can. The only one that seems remotely tricky to earn is “Sunshine “ which you get from the Omega Weapon.
Rikku’s Overdrives
Rikku’s overdrive combines items to create special attacks and powerups during battle. You have to choose any 2 items to combine; even 2 of the same kind will work. There are a lot of combinations to try. Have fun. Im not going to bother listing the whole list because its not really important and its quite tedious.