Filter Bank Multi Carrier (FBMC) based waveforms received recently a lot of interest due to their high frequency localization and hence their suitability to the fragmented spectrum utilization. In this paper, we analyze the doubly selective channel estimation aspect of the FBMC-OQAM waveform. We perform a frequency domain channel estimation using the Linear Minimum Mean Square Error (LMMSE) estimtator. In addition, we study the spreading effects of the FBMC filtering operation and the channel selectivity on the received symbol. This study is exploited later to offer a trade off between the complexity level of the LMMSE estimator and its performance accuracy. An analytical analysis of this study is provided and verified by Monte Carlo simulations.