When you first come across him, his business is still budding, and he is very eager to borrow gil from you in order to help bring it to fruition. Doing so will determine future prices for what he has to offer.

There are four places where you can lend him gil: S.S. Liki, S.S. Winno, Luca Nr 1 Dock and Mi'ihen Highroad. You should be sure to give him at least 1 gil - it will affect how much he will be inclined to help you in the future. Handing him a total of 10001 gil will drop his prices to 70% compared to other merchants, starting from Mushroom Rock Road. In other cases, his prices will be slightly to rather much steeper than retail prices, according to this chart:

There is another opportunity to adjust the pricing, as O'aka will ask you for advice in the Macalania woods. Here you are the boss - you can haggle down prices 25%, leave them as they are, or even increase prices 25%! In any case, O'aka is your merchant of choice both at the Moonflow and in Guadosalam, where the prices are really steep.

A list of the goods he has to sell, where he sells them, what abilities they have (including any empty slots) and who can equip what weapons can be found here. I compiled it from the Piggyback FFX guidebook, the best one out there I might add. Piggyback kicks Brady ass anytime.

After Mt Gagazet, O'aka's brother Wantz permanently takes over his business. The easiest way to get weapons with maximum empty slots, once you've decided that you want to get some serious remodeling done, is to buy them from him at the south entrance of Macalania Road. However, if you didn't lend O'aka any money or speak with Wantz at Mt Gagazet, Wantz will assume that you aren't interested and won't be there. Your options in that case are to fight Varunas and Adamantoises in Sin and the Omega Ruins, or buying the equipment at slightly higher prices in Kilika after the run-in with Maester Mika at Great Bridge.