Except as provided in 590:10-1-7 and 590:10-7-6 of these rules and except for the Retirement Notice and Application form, any notices, letters, payments or other documents will be considered received on the date actually received in the offices of the System, as evidenced by the OPERS date stamp, or the date of postmark on the envelope, whichever is earliest.
Okla. Admin. Code § 590:10-1-9