This paper investigates the problem of spectrally efficient operation of a multi-user uplink cognitive radio system in the presence of a single primary link. The secondary system applies Opportunistic Interference Cancelation (OIC) and decode the primary signal when such an opportunity is created. The authors derive the achievable rate in the secondary system when OIC is used. This scheme has a practical significance, since it enables rate adaptation without requiring any action from the primary system. The approximated formulas and tight lower and upper bounds for the ergodic sum-rate capacity of the secondary network are found.