I agree with what VRWC said to a point. Depending on how committed you are to upgrading within 6 months, you can start the Oscar in a 55, you just have to have the means to upgrade the tank in less than a year. A lot of this comes with your age. If you're someone living at your parents house with no income and needing parental approval, odds are there are other factors that will weigh in on this decision. If you're a adult with your own place, your own finances, and you are mature enough to handle the responsibility knowing that you'll need a larger tank soon then starting in the 55 is fine.
For a lot of people that are new to the hobby a 55 gallon tank seems huge, and it is for guppies and tetras, but when you start keeping monsters a 55 is small.