In this study, we investigate a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source. Specifically, we consider two light sources in an office space; one is the information source employed on the ceiling and the other one is a task light mounted on a desk. Our system builds upon DC biased optical orthogonal frequency division multiplexing (DCO-OFDM). The task light performs amplify-and-forward relaying to assist the communication and operates in half-duplex mode. We investigate the error rate performance of the proposed OFDM-based relay-assisted VLC system. Furthermore, we present joint AC and DC optimal power allocation in order to improve the performance. The DC power allocation is controlled by sharing the number of LED chips between the terminals and the AC power allocation decides the fraction of the information signal energy to be consumed at the terminals. Simulation results reveal that the VLC system performance can be improved via relay-assisted transmission and the performance gain as much as 6 dB can be achieved.