Since you gave him money, there are both short term and long term benefits to to his wares. Until Mi'ihen Highroad, his prices will be expensive/cheap in proportion to the amount of money you give him. After that, his prices will drop so he can help supply the people at Mi'ihen, where he sells unique weapons, including a very valuable TKO for Wakka which bears the Stone-Attack customized for Wakka. After that, his prices will drop again, when you find him in Guadosalam, where he sells Thunder-Ward/Proof equipment cheaper than the vendors at Guadosalam. After this, when you find his brother at Mt. Gagazet, Wantz sells restorative items, and some semi-useful equipment. After the battle with Sin, O'aka will have been arrested, and you find him at the Movie Theatre in Luca. Speak to him, and Wantz will be selling Tetra equipment in the Thunder Plains entrance of Macalania.

Investment for restorative items, these costs change for each area you find him (so his prices will be different in Mushroom Rock, than as if you found him in Mi'ihen.)
1-500gil, 2.0x cost of vendors
501-1000, 1.7x
1001-9999, 1.5x
10k+, .9x

The prices listed here will get you the MINIMUM prices as can be allowed at a time, and these numbers are only affected by lending money. There is another requirement, which reduces his prices based on items purchased/money lent. This number is something like 25,656 gil.
His prices are cheaper only if you buy alot of stuff from him, also selling your wares to him while he is selling cheaper items will result in your items having a lower resale value, but NOT a higher resale value if you sell items to him while having higher prices than normal vendors. This means "buy from O'aka, but never sell to".

The Tetra equipment at the end of the game has 4 customizable slots, for all 7 characters' weapons and armor, unlike what Rin sells, which has either +10% magic/strength, but the same name. Tetra equipment is also dropped from Creations at the Monster Arena.

Since you are poor, all you REALLY need to do is stop buying ANY restorative items from ANYONE. Investing 500 to O'aka is enough until you get more money. Giving him money only HELPS the 10k requirement, but its only use is to give him money if he has nothing worth selling. You can lend him money on the two boats, and one last time on the Mi'ihen Oldroad. After this, buying items is the only way to help his prices.