Hope this is of use to someone!!!

As you maybe aware, Tidus has four overdrives, but the last one is hard to come by if you don't use his others a lot.
A great way to do this is to visit the pool on the left of the Cave Entrance.
Change Tidus's overdrive mode to "Slayer", get someone (there will only be three characters to choose from at this point) to haste Tidus and make sure he does all the killing.
You will notice that even when he is using his OD, afterwards the gauge will be nearly full ready to use again!
Another up side to this is that you collect lots of power spheres which you can
use to boost up your Aeons hp.
Also you can steal some rare healing springs in here!!

Good Luck!

Another good way to get Tidus's Oveerdrive is at the monster Arena and fight Abaddon and many ofhis moves should knock you ot in one hit instantly filling it up so just revive Tidus use it and repeat, you can use around 5 overdrives in one battle with Abaddon if you're lucky.

There's already info about this in the stickies at the top of the page. :)

Thanks for sharing, though. ^^