FFX-2: Millionaire Achievement in Final Fantasy X / X-2 HD Remaster
Pay off O'aka's debt
How to unlock FFX-2: Millionaire
This achievement must be done before you reach Macalania Woods in Chapter 2. First, you must complete the "Follow that O'aka!" mission in Macalania Woods in Chapter 1 and allow him aboard the Celsius. This will add him as a vendor, and you must purchase 100,000 worth of items from him so he can pay off his debt.
A quick way to get the money needed for this can be done the first time you arrive in Guadosalam in Chapter 2. The innkeeper will offer to sell you a tip for 10,000 gil and offer a hint when he does. Save prior to speaking with him, and if the hint is "It is the last person you would expect..." then purchase the tip, if not reload your save. Once you get the correct tip, purchase the data then turn right around and sell it back to him for 100,000 gil.
Now that you have plenty of money, finish paying off O'aka's debt by purchasing items. The 100% Walkthrough mentions the cutoff for this many times, so you shouldn't miss it.