Oklahoma Attorney General Scott Pruitt this afternoon filed the state's long-anticipated lawsuit challenging the the constitutionality of the Affordable Care Act. Specifically, he filed the suit in the United States District Court for the Eastern District of Oklahoma. And the suit makes only one claim: that the minimum essential coverage requirement exceeds Congress's enumerated powers (specifically, those granted by the Commerce Clause and the Necessary and Proper Clause). You can access the complaint here.
This is now the third state-initiated suit claiming the ACA is unconstitutional, the two others being Virginia v. Sebelius and Florida v. HHS. As to why Oklahoma is not simply joining the other 26 states in the Florida case, General Pruitt has stated that Oklahoma voters' approval of State Question 756 this past November--which provides, inter alia, that no "law or rule" shall "compel, directly or indirectly, any person, employer or health care provider to participate in any health care system"--compelled him to defend Oklahoma's distinct sovereign interests in a separate legal action.