The laws on a payday loan in OK are that loan splitting is not allowed and that there are collection limits which are accepted up to a certain amount. Another law is that there will be some amount charged on the amount exceeding $100. Read more details at http://www.ehow.com/list_6748550_payday-loan-laws-oklahoma.html.
Review the terms on your agreement. It will say how much is supposed to be paid to interest and how much is paid to principal with every installment you make. You should be getting some sort of a statement of what you have paid and what balance remains owing and following it along to a zero balance.