Opportunistic spectrum access (OSA) allows unlicensed users to share licensed spectrum in space and time with no or little interference to primary users, which bring new research challenges in MAC design. In this paper, we firstly introduce the differences between the conventional multi-channel MAC protocols and the MAC protocols for OSA as well as the requirements for OSA MAC protocols, and then analyze and compare the several MAC protocols proposed so far in the literature. At last we give the characters of these protocols.