Inherent unipolar orthogonal frequency division multiplexing (OFDM) schemes have a reduced spectral efficiency (SE) when compared with direct current (DC)-biased optical OFDM (DCO-OFDM). The concept of superposition modulation was proposed as a solution to the SE loss of unipolar orthogonal frequency division multiplexing (U-OFDM) and the power efficiency loss of pulse-amplitude-modulated discrete multitone modulation (PAM-DMT). In this paper, we demonstrate that the superposition modulation can also be applied to asymmetrically clipped optical OFDM, and we refer to it as enhanced asymmetrically clipped optical OFDM (eACO-OFDM). The eACO-OFDM results in a new power and spectrally efficient scheme for intensity modulation and direct detection (IM/DD) systems. The bit error ratio (BER) performance of the proposed scheme is derived and obtained results agree with Monte Carlo simulations. The performance of the superposition OFDM-based modulation schemes is compared and presented for both additive white Gaussian noise (AWGN) and non-flat visible light communication (VLC) channels.