An opp is a rival or adversary. The word, associated with hip-hop, is generally understood as a shortening of opponent or opposition.[1]
Usage
The term can refer to a serious enemy or be applied jokingly to someone who inconvenienced you. “My opps scheduled an 8 a.m. meeting” is probably a dispute with the calendar.
Context
Who is speaking and what happened tell you more about the seriousness of the conflict than the word alone.