Warning: SPOILERS ahead!

You first meet O'aka on the S.S. Liki (I love the name of that boat ;), where he will look very interestedly at Tidus' strange clothing. After getting somewhat disappointed in the fact that Tidus has never heard of him, he'll ask for a loan to start up business as a merchant. Henceforth, he will always be a step ahead of Yuna and her party, offering rare wares for every neccessity. You can find him in the weirdest places, he even manages to sneak past restricted areas. O'aka is the only civillian to bear witness to the disastrous failure of Operation Mi'ihen, and it temporarily lowers the spirits even of this otherwise balanced and mostly worriless lad.

O'aka usually is not afraid to talk his customers over, if he sees fit. However, Yuna and her party always get a bang for their buck and dibs to the best stuff. This is because he wants to help Yuna on her journey. She reminds him of his sister, who became a summoner as well (quite possibly Lady Ginnem, he is reluctant to name her outright - perhaps because he is standing face to face with her former guardian?), and whom he was unable to help because he wasn't fit to become a guardian. After Yuna is branded a traitor, O'aka is consequently rewarded with incarceration by the followers of Yevon. He has no qualms about it and asks his brother Wantz to take on his blooming business for him, so that he can continue helping Yuna on her quest. Not that the priests are very successful in keeping a clever and gab-gifted man like O'aka at bay - you will later on find the retired merchant outside Luca Stadium.

Wantz's sneakiness and admiration for Yuna is possibly even worse than his brother's. He keeps following her during her journey, snapping shots of her and disappearing. His eagerness leads him to blow his cover at one point though - just after Yuna and her party leave Rin's travel agency at the Thunder Plains, you can get a glimpse of the paparazzo at work.

A very special thing about O'aka and Wantz is their way of speaking. In the European version, they sport a distinctive Cockney (London area) accent. I presume they speak with an Osaka or Kansai accent in the Japanese version, since they sound much like Cockney, but I haven't heard them in Japanese so don't take my word for it. I personally think that Cockney was a good choice, it's very charming and amusing in the way that you would never have expected it in a fantasy world. :)