In this paper, a novel optical index modulation (OIM) aided direct-current (DC) biased optical OFDM (OIM-DCO-OFDM) scheme for visible light communication (VLC) systems is proposed. Different with traditional index modulation schemes, the proposed OIM-DCO-OFDM scheme uses index bits to determine which subcarrier transmit original signals and which subcarrier transmit conjugate signals. The constellation of traditional phase-shift keying (PSK) and quadrature amplitude modulation (QAM) is symmetrical about the real axis. It means that we cannot distinguish the original signals and conjugate signals. In order to recover index bits at the receiver, we propose a unipolar pulse amplitude modulation (PAM) scheme for the modulation of constellation bits, and design a zero-forcing (ZF) based detector. Compared with dual-mode index modulation aided DCO-OFDM (DM-DCO-OFDM) scheme, the proposed scheme has 1 dB and 2 dB performance gain at the bit error rate (BER) level of 10- 4 when the spectral efficiency is 1.21 bits/s/Hz and 2.18 bits/s/Hz, respectively. The proposed OIM-DCO-OFDM scheme can achieve 80% spectral efficiency improvement than the DM-DCO-OFDM scheme at the modulation order of M= 4.