Why it could be him:
- He hates the Lannisters. In the premiere, Oberyn makes it very clear to Tyrion that he has reasons to seek revenge against the Lannisters: Tywin's bannerman The Mountain raped and killed Oberyn's sister, Elia, before murdering her children.
- Within hours of his arrival at King's Landing, Oberyn shows he isn't afraid to get violent, stabbing his dagger through a man's wrist for singing the Lannisters' song at the brothel.
Why it couldn't be him:
- Oberyn gets into a passive-aggressive argument with Tywin and Cersei Lannister at the wedding, but he doesn't approach the head table or have any direct interaction with Joffrey.
- Tywin confronts Oberyn in the brothel about having studied poisons, a pretty strong case against him. Though Tywin makes a power play by involving Oberyn in Tyrion's jury, Oberyn could still be a suspect.