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We demonstrate that optical OFDM (O-OFDM) based on discrete Hartley transform (DHT) can be used with large-size constellations, furnishing alternative simplified schemes suitable for intensity-modulated direct-detection (IM/DD) systems. Both DC-biased and power-efficient asymmetrically clipped (AC) solutions are analyzed for real constellations from BPSK (binary-phase-shift keying) to 32 PAM (pulse-amplitude modulation). The performance is compared to standard O-OFDM based on fast Fourier transform (FFT) using constellations from 4 QAM (quadrature-amplitude modulation) to 1024 QAM, showing perfect agreement. The analysis of clipping effect evidences the suitability of the proposed AC scheme for adaptive systems.