In this paper we address the problem of fully distributed assignment of users to sub-bands such that the sum-rate of the system is maximized. We introduce a modified auction algorithm that can be applied in a fully distributed way using an opportunistic CSMA assignment scheme and is $\epsilon$ optimal. We analyze the expected time complexity of the algorithm and suggest a variant to the algorithm that has lower expected complexity. We then show that in the case of i.i.d Rayleigh channels a simple greedy scheme is asymptotically optimal as $\SNR$ increases or as the number of users is increased to infinity. We conclude by providing simulated results of the suggested algorithms.