Due to integrated positive features of both hypercube and tori, optical multi-mesh hypercube (OMMH) networks in high-performance computers are regarded as a class of promising optical inter-connection networks. This paper firstly derive that the diagnosability of OMMH under the pessimistic strategy is (2n+6)/(2n+6), which shows that the OMMH possesses strong self-diagnosingability. With the improved cycle decomposition method by Yang in J. Parall. Distrib. Comput. [10], a fast diagnosis algorithm to identify all faulty nodes tailored for OMMH, which runs in O(Nlog2N) time is also proposed, where N is the number of the processors of an OMMH.