Opportunistic Spectrum Access has recently emerged as a set of approaches that seek to minimize the problems of scarcity and inefficient use of radio spectrum caused by its current policies of administration and allocation. This paper presents some modifications and adjustments made to the Opportunistic Spectrum Access model developed by Urgaonkar, R. and Neely, M. (2008). We evaluate the applicability of this model in realistic primary wireless network scenarios deployed in the present (particularly, GSM in coexistence with DVB-T). We also introduce two additional optimization objectives that define new criteria of resource allocation: Proportional Fairness, which considers the particular requirements of each secondary user and maximizes the total system occupancy, and Max-Min Fairness, which sets a fair policy to distribute the spectrum availability in order to maximize the minimum value of throughput guaranteed to all secondary users.