O'Brien (forearm) will throw a bullpen in the next day or two, Lynn Worthy of the St. Louis Post-Dispatch reports Tuesday.
O'Brien was shut down from throwing June 23 after reporting pain in his forearm, but he's resumed a throwing program and has a bullpen session scheduled. The 29-year-old has been on the IL since March 31 due to a flexor strain in his right forearm.