I've been breeding oscars for decades and I can't tell(I could not even tell my pairs until the females tube dropped) and I'll go further to say that anyone who claims to be able to sex a oscar visually is full of kaka. The only way to properly sex a oscar with a 100% certaintly is to watch them lay eggs for females and fertilize eggs for males.
How to Sex an Oscar Fish - Male or Female? - Oscarfishlover.com
Oscars do not reach sexual maturity until around 12-16 months on average which puts them around 9-10". I always used a 75g for when the oscars when they were ready to spawn then they were moved back into a 150g when the deed was done. Even with a pair in a 75g was a huge amount of effort, this required 2-3 50% weekly water changes to keep the nitrates below 20ppm.