On 18 June 2012, Olejnik signed a three-year contract with Championship side Peterborough United for an undisclosed fee. Olejnik was a replacement for both Joe Lewis and Paul Jones, who left for Cardiff City and Crawley Town respectively. Olejnik made his debut for Peterborough United on 14 August 2012 with a clean sheet in a 4–0 win over Southend United in the cup. He made his Championship debut against Leicester City six days later. Manager Darren Ferguson was later quoted saying "Bobby's a really good goalkeeper. The save he made in the first-half was outstanding, but he also gives us something we haven't had from our goalkeepers in recent seasons and that's excellent communication skills".