With the explosive growth of spectrum demand for various wireless devices and applications, it is more and more difficult to allocate contiguous spectral bands for radar and communication nowadays. In this paper, the problem of dual-functional radar-communication (DFRC) multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) waveform design under non-contiguous spectrum bands is studied. We consider a flexible spectrum structure where a large number of non-contiguous bands are used for communication and some idle bands which are complementary to the communication bands are exploited for radar detection. The symbols transmitted on the communication bands are first designed by minimizing the multi-user interference (MUI) to enhance the performance of communication. Meanwhile, the radar performance is improved by designing the waveform of idle bands. We formulate the radar beampattern design as a rank-1 approximation problem, which can be solved with a closed-form solution. Numerical results show that we can achieve omnidirectional strict beampattern design and data transmission with zero MUI.