Please don't use oxymetholone. It is just not worth it, the drug will fuck up your HPTA to an extreme degree even if you're a fully grown man, aged 30. You don't want to experiment with this drug, mate, particularly because of your age...You will feel great while on it, and you will get huge, but because of the decrease in HPTA function, when you finish it you will lose the mass you gain VERY quickly. More than likely you will also suffer from depression and fatigue. No joke, stay away from oxymetholone at your age.
IF you're anything like I was at your age, very stubborn, and I still am (at 22), then you won't heed our advice. But know this, I was thinking about gearing at your age, then was told by some older more experience folk its not a good idea and I should wait until I'm AT LEAST 22, and I took their advice.
In the gym I naturally gained a lot of strength and have not plateued yet, so I don't think I'll gear until I do plateau. Anyway, sorry about the rant, but just don't experiment with your body at your age, that's all.