Cognitive Radios (CR) technology gives network operators more opportunities to create new communication services by sharing under-utilized spectrum owned by the licensed users. Despite of the advantages from CR, since secondary users should protect the primary service during their communication, the channel access of the secondary user is very restricted and opportunistic. This restriction occasionally makes the secondary users hard to meet the required QoS of them in some situations. As a resolution of this performance restriction, in this paper, we propose a scheme to create additional transmission opportunity for the secondary users and efficiently allocate the opportunity to maximize the network throughput considering two overlaid wireless ad hoc networks. Using the acquirable geometric and statistical information about the primary system, additional transmission opportunity is created by means of a power control and estimated as the opportunistic rate. The opportunistic rate is optimally assigned to the flows among the secondary users to maximize the throughput with an optimization method. Simulation results show that the proposed opportunity estimation and allocation scheme considerably improve the secondary network performance without degrading the primary service quality.