U.S. oil output could start to take a hit by late 2015, OPEC said on Monday, suggesting the exporter group will have to wait beyond its next meeting in June to see if the oil price collapse is beginning to dent the shale oil boom. The halving of oil prices since June 2014 has prompted spending cuts by oil companies and a drop in U.S. drilling, raising expectations of slowing output in countries outside the Organization of the Petroleum Exporting Countries (OPEC).