Digital nonlinearity compensation (NLC) schemes such as digital backpropagation and Volterra equalization are well known to be effective techniques in mitigating optical fiber nonlinearity, thus offering improved transmission performance. Alternatively, optical NLC, and specifically optical phase conjugation (OPC), has been proposed to relax the digital signal processing complexity. In this paper, a novel hybrid optical-digital NLC scheme combining OPC and a Volterra equalizer is proposed, termed Volterra-Assisted OPC (VAO). It has a twofold advantage: it overcomes the OPC limitation in asymmetric links and substantially enhances the performance of Volterra equalizers. When NLC is operated over the entire transmitted optical bandwidth, the proposed scheme is shown to outperform both OPC and Volterra equalization alone by up to 4.2 dB in a five-channel, 32 GBaud PM-16QAM transmission over a 1000 km EDFA-amplified fiber link. Moreover, VAO is also demonstrated to be very robust when applied to long-transmission distances, with a 2.5-dB gain over OPC-only systems at 3000 km. VAO combines the advantages of both optical and digital NLC offering a promising tradeoff between performance and complexity for future high-speed optical communication systems.