How did Carter lose OK?

[Fuzzy Bear]

Oklahoma had a white, Southern Democratic-type electorate in 1976, but many of those Democrats had long stopped voting Democratic in Presidential races, and Oklahoma had fewer black voters to narrow the gap then other Southern states.  Other than 1964, with LBJ, Oklahoma hadn't gone for the Democrats since Truman in 1948.

[Fuzzy Bear]

In addition to what others have said, northern Oklahoma (especially near the Panhandle) is more ancestrally Republican (and a lot less Southern) than the rest of the state, and when combined with the votes of (white) conservatives from the growing OKC and Tulsa metros in 1976...well, Ford's victory is fairly easy to explain.

Note that Carter still won many of the more Southern/Dixiecratic counties of southeast OK in 1980 while losing the rest of the state.

Oklahoma was a very, very white (88%) state in 1976, with a black population of about 7-8%.  The Native American vote in Oklahoma was not a monolithic Democratic vote for President. 

Oklahoma moved toward the Democrats in 1988 due to the 1980s farm recession, but this is now ancient history.