According to the Oklahoma Bar Association:
Q: Can my employer fire me without a good reason?
A: Oklahoma has traditionally recognized the “at will” doctrine, meaning that an employee works and a business employs on an “at will” basis, and either may cease the employment relationship at any time. However, the ability to terminate an employee “at will” is limited under some circumstances. For example, an employer cannot terminate an employee in retaliation for the employee’s exercise of rights under Oklahoma’s Workers’ Compensation Law, or base a decision to terminate on an employee’s race, color, sex, pregnancy, age (40 and over), national origin, religion, genetic information, or mental or physical disability or in retaliation for reporting the employer’s illegal conduct.
If you have an employment contract, you should refer to your contract to determine for what reasons you may be terminated. If you have a contract stating grounds for termination, your employer is not able to terminate you at will, but is required to terminate you only for the reasons stated in your employment contract, or they become liable for damages.
Even if you do not have a signed employment contract, you should refer to your company’s employee manual or handbook to see what it says about the reasons for which you may be fired, grievance procedures or other terms of your employment. Employers may be required to adhere to statements made in such publications.