Can an ox kill a cape buffalo?

That depends on the size of the ox and the size of the buffalo. Oxen can grow to be much larger than a cape buffalo (like those of the breed Chianina, Charolais or South Devon), whereas the standard weight of a bull cape is only around 2000 lbs. Both animals are capable of growing sharp horns used to defend themselves with, and both are very powerful as far as strength is concerned.

But with all that it is likely that an ox nor a cape would purposely kill another animal that eats the same food source (grass) as the other. They will definitely fight for dominance, oh yes, but it's rare for such fights to end in death. Quite often the larger animal is the one that will win and the younger or smaller one will become submissive to the larger animal. Oxen are castrated males too, which means they generally will have a more milder temperament than a bull cape, which makes it less inclined to want to kill another bovine-cousin.