U.S. oil drilling has seen its best quarter since crude prices tumbled two years ago mainly due to small operators returning to the well pad, but analysts say the continued recovery in the rig count depends on whether OPEC's output reduction plan can bring the market back to $50 a barrel. Since May, drillers have added 100 oil rigs with the rig count rising to 416 rigs last week, that included the most additions in a quarter since the first quarter of 2014, according to oil service firm Baker Hughes Inc. The number of rigs had plunged from a record high of 1,609 in October 2014 to a low of 316 in May after crude prices collapsed in the biggest price rout in a generation, in part due to U.S. shale producers adding to a global oil glut.