Oklahoma constitution
Article XIV: banks and banking
Section 2

The Legislature shall have authority to classify loans and
lenders, license and regulate lenders, define interest and fix
maximum rates of interest; provided, however, in the absence of
legislation fixing maximum rates of interest, all contracts for a
greater rate of interest than ten percent(10%) per annum shall be
deemed usurious; provided, further, that in contracts where no
rate of interest is agreed upon, the rate shall not exceed six
percent (6%) per annum.