OFDMA technology can significant improve the transmission reliability in multi-user communication systems because of its inherent frequency diversity. In a recent work, we have derived a surprising result which demonstrates that OFDMA systems can achieve a frequency diversity gain which is equal to the total number of independent subcarriers. In this paper, we shall show that the frequency diversity and the frequency multiplexing can be simultaneously achieved in OFDMA systems with a fundamental tradeoff between them. The random bipartite graph theory is used to model and analyze this diversity-multiplexing tradeoff problem. In particular, the maximum proper f-matching is introduced as a subcarrier allocation method which can minimize the user outage probability with fairness assurance given some multiplexing requirements. Similar to the Zheng-Tse tradeoff in MIMO systems , the optimal diversity-multiplexing tradeoff in multi-user OFDMA systems and it will be shown that its curve can be characterized by a piecewise linear function, despite of the user conflicts in the subcarrier allocation.