Due to the difficulty of coordination in multi-cell random access, it is a practical challenge how to achieve the optimal throughput with decentralized transmission. In this paper, we propose a decentralized multi-cell aware opportunistic random access (MA-ORA) protocol that almost achieves the optimal throughput scaling in a multi-cell random access network with one access point (AP) and multiple users in each cell. Under our MA-ORA protocol, each user opportunistically transmits with a predefined physical layer data rate if the desired signal power to the serving AP is sufficiently large and the generating interference power is sufficiently small. As a main result, it is shown that the proposed MA-ORA protocol can achieve both the almost full multiplexing and multiuser diversity gains in a high signal-to-noise ratio regime if the number of users scales faster than a certain level. Our results are validated by computer simulations.