Okla. Admin. Code § 710:90-1-4 – Taxpayer identification number; consolidated returns

(a) The employer's identification number for Oklahoma Withholding Tax purposes will be the same number assigned by the Internal Revenue Service for reporting Federal employment taxes. Every employer must have a Federal Employer Identification Number.
(b) Commission files will be maintained by the taxpayer identification number (Federal Employer Identification Number); any correspondence relating to withholding should refer to this number to expedite the processing of returns and the crediting of payments.
(c) Only one valid taxpayer identification number may be assigned to an employer. A new Federal Employer Identification Number should be obtained and registered with the Commission when any of the following changes occur:
1. (1) A sole proprietorship incorporates;
2. (2) A sole proprietorship takes in a partner and operates as an partnership;
3. (3) A partnership incorporates;
4. (4) A partnership is taken over by one of the partners and is operated as a sole proprietorship;
5. (5) A corporation changes to a partnership or to a sole proprietorship;
6. (6) A sole proprietorship purchases or inherits an existing business (the employer identification number of the former owner cannot be used even if he or she is a spouse);
7. (7) A partnership is terminated and a new one is begun; or
8. (8) An estate operates a business after the owner's death.
(d) Consolidated returns of two or more employers representing separate legal entities are not permitted unless written approval has been granted by the Tax Commission even though one employer may own a controlling interest in another.
(e) An employer whose operation consists of multiple locations or payroll centers may request permission to report separately when centralized reporting causes undue hardship. Approval will result in the assignment of a unique taxpayer identification number consisting of the Federal Employer Identification Number and a three digit control number. Until an employer receives written approval to report as either "consolidated" or "separately," any returns should be filed as otherwise required.