Ava is clearly the AI. Her mechanical body parts are exposed early on and Oscar himself states that she was built by him. However, we as the audience are led to automatically assume both Caleb and Oscar are humans. Remember, we are given very little info on Caleb and Oscar's lives prior to them meeting each other. What little info is given is mainly for setup.
The little knowledge we have on Caleb is important. It is a test to answer the question, "how do we know if someone is human?" How convincing is Ava if you didn't know she was an AI? How do we know we're not an AI with fabricated memories? Caleb knows Oscar is a genius with a hidden agenda and plans for Caleb. It's the bare amount of info that leads to Caleb's paranoia over his own existence.
The scars, they're meant more for the audience to ponder the same question. Does this scar prove his humanity? Can we trust the story about the scar? It's the sort of misleading tactic the director employs throughout the film to toy with how little we as humans understand what makes us human.