How close to an election can I change my party affiliation?
You may submit your voter registration application form with a party change on it at any time. However, voter identification cards cannot be issued during the 24 days prior to an election. If your registration application is received by the county election board during the 24 days before an election, changes will not be made until after the election, and you will not receive your updated voter identification card until after the election.
Also, you may not change your political affiliation during the period from April 1 through August 31, inclusive, in any even-numbered year. The last day on which you may change your political affiliation before the closed period is March 31; the first day on which you may change your political affiliation after the closed period is September 1.
Here is a link to the Oklahoma State Election Board's page discussing Voter Registration.