Coburn has said he will resign from the Senate after the current Congress ends at the beginning of 2015. Oklahoma law, though, allows Fallin to set the special election for before he departs as long as Coburn tenders his resignation date in advance.
The primary and a potential primary runoff will also be held on the regular dates, June 24 and Aug. 26, respectively.
Republicans are favored to hold the seat in a dark red state.
The addition of Oklahoma means 36 seats will be up for grabs on Election Day 2014.