Oklahoma does not follow the so-called “convenience of the employer” rule that allows states to tax nonresidents who work remotely so your son should not pay OK tax for teleworking from Texas.
As the previous answer said, OK nonresidents are only taxed on salaries, wages and commissions for work performed in Oklahoma.
More to point, a private letter ruling from the Oklahoma Tax Commission dated June 20, 2014 says: As a nonresident whose salaries, wages and commissions are for work performed outside of Oklahoma, and who derives no income from sources within the state of Oklahoma, it is the opinion of the Tax Policy Division that your employer is not required to deduct and withhold Oklahoma withholding tax from your wages.
Private letter rulings are not tax policy as they are in response to a specific case. However, they have been used in defense of tax positions.
You can file a nonresident OK return and only report the wages from Oklahoma State University. You cannot notate anything with an e-filed return. You can file by mail and include an explanation. Or you can e-file and wait for a possible notice from the Oklahoma Tax Commission in which case you can file a response.
Your son should also file a OK W-4 to stop withholding.
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