Senators question ObamaCare hiring

But they questioned whether all of those jobs truly represented the "critical needs" for which direct-hire authority is usually reserved.

"Please explain why there was a 'critical need' for HHS to waive provisions of federal hiring law to hire 90 dental officers to implement the health care law," they wrote.

The letter to OPM also says HHS would have had to hire 10 people per day to fill all of the positions OPM designated as critical.

"The pace of hiring that would be required for HHS to maximize this authority and fill each of those 'critical needs' is incredible," the lawmakers wrote.

In addition to Coburn, the letter questioning HHS's hiring authority was signed by Sens. Richard Burr (R-N.C.), Michael Enzi (R-Wyo.), Charles Grassley (R-Iowa), Kelly Ayotte (R-N.H.) and John Thune (R-S.D.).