My Oscar was about 1.5 inches in April last year. He's about 8" now and probably be about 9" next month. But I haven't been feeding him for fast growth
My 2 are getting girth now they are 'of age' (about a year or a bit older). 12+ inches for both, feeding once - twice daily 74* water temps and reasonably clean water...50% WC once a week on all heavily filtered tanks.
Our Oscars have been doing quite well on beefhearts and bloodworms. Ours were about an inch and a half when we got them in Oct and now they are probably 6-7 inches now. The fish really started to get bigger once we gave them the beefhearts.
Basically they grow 1 to 1.5 inches in a month maybe two with the right diet and very high water quality until they hit about 8 to 9 inches then they start growing slower but most O's will hit 10 to 12 inches at a year old if properly cared for