Orthogonal time frequency space (OTFS) is a promising candidate for high-mobility communications that has the potential to overcome the challenges brought by severe Doppler effects. In this letter, we investigate the optimal power allocation (OPA) for OTFS transmissions employing frequency-domain linear equalizers, including frequency-domain zero-forcing (FD-ZF) and/or minimum mean squared error (FD-MMSE) equalizers. More specifically, for FD-ZF, we derive a closed-form expression for the received signal-to-noise ratio (SNR). Leveraging this, we develop the OPA method under using FD-ZF by applying the Lagrange multipliers to solve the optimization problem of maximizing the received SNR. Similarly, for FD-MMSE, we give a closed-form expression of its signal-to-signal-plus-interference-plus-noise ratio (SSINR), and obtain the OPA to maximize the SSINR. Our analytical results show that, more transmit power will be allocated to the poorer subchannels in time frequency (TF) domain of OTFS, which is contrary to the water-filling (WF) method for orthogonal frequency division multiplexing (OFDM).