With only nine minutes on the clock Ricky Holmes burst into the box to put the home side ahead but Matt Harrold headed home after half-time
Rovers pushed for a winner but O'Brien ensured both sides had to settle for a first point of the new campaign.
After their 3-0 opening day drubbing at Port Vale, Holmes kick-started Barnet's season when he turned Adam Virgo with ease before racing forward and slotting home into the bottom right corner.
But after the restart Rovers came out of the traps fastest and in the 55th minute Harrold planted in a free header from a corner.
Rovers immediately began to pepper the Bees' goal with O'Brien forced into smart saves from Eliot Richards and Wayne Brown just after the hour to ensure the scores stayed level.