Radio spectrum is one of the most scarce and valuable resources for wireless communications. Cognitive radio has been considered as an efficient means to opportunistic spectrum sharing between primary (licensed) users and cognitive radio users. In this paper, based on the two-phase channel and power allocation scheme proposed by A. T. Hoang etc., we present an opportunistic spectrum access approach for cognitive radio network. In the scheme proposed by A. T. Hoang etc., they consider a cognitive radio network that consists of multiple cells and the system throughput is defined as the total number of subscribers that can be simultaneously served. In this paper, we consider a cognitive radio network as self-organizing network. Furthermore, the throughput is defined as the average probability of success transmission. In our proposed approach, for each available channel, TDMA frame consists of N time slots, and each active cognitive user is assigned one transmission slot different from those of other active cognitive users in each frame. It allows an active cognitive user utilize the slots pre-assigned to the other active cognitive users under a range of values for accessing opportunity. We evaluate the performances of the opportunistic spectrum access approach in view of system throughput and energy efficiency.