In this letter, layered discrete Hartley transform (DHT)-spread asymmetrically clipped optical-orthogonal frequency division multiplexing (LDHTS-ACO-OFDM) is presented; it uses DHT for multiplexing/demultiplexing and pulse-amplitude modulation (PAM) alphabets. LDHTS-ACO-OFDM yields several concrete enhancements over classical layered asymmetrically clipped O-OFDM (LACO-OFDM), such as low peak-to-average power ratio (PAPR), lower computational complexity, improved bit error rate (BER), and lower optical power penalty in a dispersive channel. Besides, LDHTS-ACO-OFDM augments the spectral efficiency relative to DHTS-ACO-OFDM with each addition of layer and ousts the inefficient bandwidth utilization limitation of DHTS-ACO-OFDM.