It sounds like you have Acetaldehyde which is a compound that produces green apple flavor and aroma in homebrew, and is a by product of fermentation. Acetaldehyde is formed during the conversion of sugar to ethanol by yeast.
Some of the common causes for an incomplete fermentation include not pitching enough yeast up front, yeast that starts out in poor health, failing to aerate your wort which will impede healthy yeast growth and also halting your fermentation too soon. You can also get the “green apple” flavor if you have poor temperature control during fermentation which can drive excessive acetaldehyde production or prematurely stop fermentation. Finally you can get it by pitching finings too soon – which can force the yeast to flocculate out before fermentation is complete.
Oxidisation is usually a wet cardboard taste with a darkening of the beer sometimes going to a mucky brown in colour