Why is oxygen excluded from the catalytic cracking of hydrocarbons?
You mean catalytic cracking of long chain hydrocarbons? There is a very good reason to exclude oxygen.
Hydrocarbons are unstable with respect to oxidation. The oxidation products, carbon dioxide and water, are thermodynamically stable species. Carbon-carbon skeletons need to be preserved in the cracking process so that they can continue to be used as fuels.