Yes, it is an oxymoron. Christianity and Objectivism are two completely different systems. In Objectivism, reason is an absolute and rejects all forms of mysticism (belief in the unknown). Also, it is pro-selfishness, in Christianity you can't do something simply because it makes you happy, you have to do because it makes God happy. To call yourself an Objectivist or a Christian or a Socialist, etc., you have to follow the core principles.
That being said there are people who call themselves Christians and believe in God, but also rely heavily upon reason as a guide in their lives and agree with many aspects of Objectivism.