O'Brien (calf) is scheduled to throw a bullpen session Tuesday, Jeff Jones of the Belleville News-Democrat reports.
It will be the first time O'Brien has thrown from the mound since he went down with a minor right calf strain a week ago. The injury is expected to prevent the reliever from pitching for Team South Korea in the World Baseball Classic, but O'Brien should not have trouble ramping up in time for Opening Day if he can avoid setbacks.