We propose a low-complexity and high-performance detector for multi-input multi-output circular filter bank multicarrier (C-FBMC) system on the basis of an orthogonal approximate message passing algorithm. Low complexity and high performance are realized through the use of a special structure of the modulation matrix and block diagonalization of the equivalent channel matrix in the frequency domain. After exploring the property of the C-FBMC modulation matrix in the frequency domain, we design an algorithm to replace the direct inversion of the matrix, thus reducing complexity considerably. The complexity per iteration of the proposed method is O(KMT log₂ M), where K, M, and T denote the number of subcarriers, subsymbols, and transmit antennas, respectively. Numerical simulations show that the proposed iterative algorithm is superior to conventional detectors in terms of performance and computational complexity.