The answer to the "is this legal?" question is more complex than simply "private versus public" university. There are fairly strict federal anti-wiretapping laws. It is hardly clear that a private university may eavesdrop on students' phone or computer conversations, even if conducted over the university's network. Furthermore, many states have their own anti-wiretap and anti-eavesdrop laws, which are even stricter than federal standards. I don't have the time nor the inclination to do research into Oklahoma law, but we did some research into this area of state and federal law for the LaMacchia case and concluded that in its investigation of David LaMacchia, MIT very well might have violated federal laws.

It is definitely legal for private entities to have strict control over their own resources. In particular, it is legal for a company to listen in on their phone conversations for quality control purposes if they announce this fact ahead of time (you may have heard such a disclaimer when you call tech support, for instance). Also, if a student signs such a waiver acknowledging that the situation at this school is not necessarily that of the "norm" or of potentially overriding state or federal law, there is a good chance the student has signed away their right/priviledge of absolute privacy. Unless there is serious violation of Constitutional rights or if the contract is constructed or negotiated in bad faith or in some illegal manner, the old saying applies: "a contract is a contract". Ern