Ostlere graduated from the Royal Academy of Dramatic Art in 2008. He prepared for playing a doctor in Holby City by going to a hospital with Ty Glaser and spending the day with a consultant and registrar. He also watched surgery being performed and followed F1 doctors.

In 2011 Ostlere appeared in the series premiere of the American television series Game of Thrones, appearing as Night’s Watch ranger Waymar Royce. He is killed by a White Walker six minutes into the episode’s cold open, giving him the distinction of playing the first character ever killed on that show, which is known for its large number of character deaths. On this, Ostlere comments, “To be honest, when you do those smaller parts, you’re just happy to see yourself up there because sometimes those things can get cut”.