I don’t believe cheating exist in an open relationships. But in monogamous relationships there’s are boundaries which each partner must respect.

Saving someone’s life can be an expection but what happens after the critical incidence is where the cheating comes in, if boundaries are crossed.

Oral sex is cheating when you feel the same arousal and ecstasy from sex, or there’s an attraction that may lead to subsequent feeling of pleasure. At least that’s my version of cheating.