This paper analyzes the performances of indoor optical wireless data transmissions based on unipolar orthogonal frequency division multiplexing (OFDM). In particular, it is shown that using frequency-domain pre-equalization can provide benefits in terms of the reduction in the required optical transmit power for a given desired bit error rate (BER) from uncoded transmissions. Known for its power efficiency, asymmetrically clipped optical OFDM (ACO-OFDM) is considered as a unipolar modulation scheme for intensity modulation with direct detection (IM/DD). In addition, flip-OFDM is also considered as an alternative unipolar modulation scheme which is known to be as power efficient as ACO-OFDM. For both ACO-OFDM and flip-OFDM, analytical and simulation results show that using pre-equalization can save up to 2 dB of transmit optical power for a typical indoor optical wireless transmission scenario with the bit rate of 10 Mbps and the BER target of 10-5.