O'Neill (back) is scheduled to begin a rehab assignment Thursday, John Denton of MLB.com reports.
The Cardinals had previously identified Tuesday as the soonest O'Neill could begin playing rehab games. The new start date would seem to set him up for a possible return early next week as the team begins a road trip, although that hasn't been clarified. O'Neill has been out since May 4 with a lower-back strain.