This paper describes parameters design of multiuser OFDM on doubly selective fading channels. It is well-known that OFDM is vulnerable to time selectivity due to the extension of symbol period, generating inter-carrier interference (ICI). In this paper, we first derive the closed form of signal to interference plus noise ratio (SINR) for single user OFDM with a linear time-varying model. In multiuser case, we propose several design criteria depending on the channel environments. We also discuss an effect of subcarrier allocation on the parameters optimization in multiuser access systems.