I live in Oklahoma and my boss makes us take our PTO and does not allow us to save this, for example if planning to have a baby or special vacation. If there is a holiday my boss requires that full time employees take up to 40 hours in that week so that we do not accumulate to much PTO, even though 32 hours is considered full time. Our PTO is also our sick leave, holiday and vacation and should be at our judgement when we have to use it or not. Is this legal?

PTO does not have to be offered at all, therefore, it is within the employer's legal right to control when it is used. It is a myth that paid time off can be used at the employee's discretion if the employer disagrees. I'm not saying it's fair, I'm just saying it's not illegal in your state.