Oz glanced over at the person who was now sharing his bench. The fact that she didn't ask "what are you doing?" or said something about imaginary guitars, and the mental state of people who played them meant he felt rather kindly disposed towards her. Because playing an invisible guitar -- well it wasn't something that Oz necessarily wouldn't have done out in the world that he had come from.
(In fact young Oz had been known to play quite the awesome air guitar)
"Not a song. Just chords actually." He moved his fingers through a few more positions before releasing his grasp on the imaginary instrument. He didn't carry things further by putting "away" the imaginary instrument. He just stopped trying to play those imaginary chords. "Keeping my fingers in shape."
All told, these were the most words Oz had spoken since he had woken up here. He was being downright chatty.