Unlike most people in Spira, O'aka speaks with a notable Cockney accent. A few other known characters that speak with a certain accents are Wakka, Wantz, Rin, and Seymour Guado, though their accents are native to the world of Spira.
O'aka has floppy red hair with a small black hat perched atop it, secured with string under his chin. He wears a short brown waistcoat that reveals his stomach under a blue shirt, with baggy beige trousers, open sandals and gloves. He also wears his pack of wares upon his back.
O'aka is at first pompous and quite rude, commenting on Tidus's clothes and income the first time they meet, but he proves himself to be a valuable asset to the party, being there when they need him and providing occasional discounts. He can be sneaky, managing to slip past guards and security, and his motives to help Yuna and the party are rather close to his heart.
Despite the disagreements they get into, it seems O'aka does indeed care for his brother Wantz.
O'aka first appears in Besaid village, though the player does not officially make his acquaintance until sailing on either the S.S. Liki or S.S. Winno depending on if the player goes below the decks on the way to Kilika or not. When the player first encounters O'aka he comments on Tidus's clothes and mocks him by saying he looks like he has not got much money. After this, he introduces himself as a merchant. O'aka appears in a multitude of locations afterward, including Luca and Mushroom Rock during Operation Mi'ihen where he seems to have been able to sneak past the guards.
Surviving the battle, he finds his way to the Moonflow, and later to Macalania. He is also seen using a sphere before running away from guards after the player kills Seymour Guado and his accomplice guards. He is once again seen in Bevelle before the party fights Seymour for the second time. O'aka's brother, Wantz reveals somewhat later in the game he has been captured by Yevon for conspiracy - namely for supplying goods to Yuna and her party with full knowledge of Maester Seymour's recent death. O'aka later escapes and returns to Luca where he spends the remainder of the game after pledging to leave the merchant life.
Among several of his conversations, O'aka also mentions his sister had tragically died during her duties as summoner.
O'aka returns in Final Fantasy X-2. At the beginning of the Eternal Calm he bought the Macalania Travel Agency from Rin, but when Macalania Temple sank to the bottom of the lake, O'aka found himself with few customers to sell to, and no money to pay the Al Bhed for the agency.
The Gullwings can offer to help O'aka by stowing him away on the Celsius and buying all his merchandise so he can pay his debts. Once the debt has been paid, O'aka returns to the agency and offers some nice accessories for an affordable price. On the other hand, if O'aka is turned in to the Al Bhed, he will later be found working his debt off in the Bikanel Desert.
The player can talk to O'aka numerous times on the CommSphere and can help him in the mission to save the travel agency from monsters. If the player follows through with this, O'aka reconciles with his brother Wantz and they start manning the shop together. O'aka can also appear in the hot springs area in Mt. Gagazet in a CommSphere event.
At O'aka's first four locations, the player has the opportunity to give him gil to start up his business.
The amount of money the player gives O'aka determines the prices in his shops in the following manner:
0 - 100 gil -> all items/weapons will get 100% increase in prices.
101 - 1000 gil -> all items/weapons will get 50% increase in prices.
1001 - 10000 -> all items/weapons will get 20% increase in prices.
10001+ -> items/weapons will get 30% discount on all prices.
The player can also affect O'aka's prices at the Macalania Woods - Lake Road location, where if the player enters and then immediately leaves the buying menu without purchasing anything, O'aka asks the player to judge his pricing level. Replying that they are too expensive drops prices by 25%.
O'aka's debt changes based on the game mode. If playing New Game, his debt will be 100,000 gil. In New Game Plus, however, it has been reduced to 1,000 gil. He will remain on the airship until Chapter 3, even if his debt has been paid off. His prices will be greatly reduced to the point where the player can buy cheap items from him and sell them to Barkeep for a profit.