We consider the pilot symbols for orthogonal frequency-division multiplexing (OFDM) over doubly selective channels. Locations, powers, and phases of pilot symbols play important roles in the channel estimation. We attempt to minimize the mean-square error (MSE) by selecting powers and phases of pilot symbols. In this paper, based on the MSE-optimal pilot symbols for time-selective and frequency-flat channels, we obtain the pilot symbols for doubly-selective channels. Through simulations, we show the channel estimation performance with the proposed pilot symbols compared to those with the equi-powered pilot symbols and the optimal pilot symbols.