We propose and demonstrate a simple but accurate time-domain method to measure the carrier-to-signal power ratio (CSPR) of optical single-sideband (OSSB) signals. In this method, we estimate the CSPR using the mean and standard deviation of asynchronous amplitude histogram obtained from an oscilloscope equipped with DC-coupled photo-detector. We provide a closed-form expression on the CSPR using these statistical averages. By using numerical simulation, we first assess the validity of our theoretical expression for five different kinds of OSSB signals. Next, we carry out experiments using 100-Gb/s orthogonal frequency-division multiplexed signals to evaluate the accuracy of the proposed method. We show that the proposed method makes the estimation errors less than 0.9 dB over wide ranges of CSPRs and optical signal-to-noise ratios. The accurate CSPR estimation of the proposed method regardless of the frequency gap between the optical carrier and information-bearing signal is also demonstrated.