Why does Oklahoma have so many registered democrats?

[morgieb]

The state doesn't appear to have an ancestral democratic domination like many areas in the south, so why are there so many registered democrats there?

The simplest explanation is the rule of party registration itself. Most of the South don't have party registration, Oklahoma (and Louisiana, which also has a very high amount of registered Democrats) do and therefore still have lots of registered Democrats who cbf changing their registration.