Next-generation passive optical networks (NG-PONs) promise significant bandwidth increase to end users. Research and standardization efforts are underway to enable enhancements from contemporary PONs to NG-PONs. An area that has not been much investigated from the context of NG-PON2 (as defined by the Full Service Access Networking [FSAN] body) is the medium access control. We showcase for the first time the open problem of MAC implementation beyond WDM PONs, in NG-PON2 with multiple-line-rate channels. We propose a solution called OLIMAC, or the Open Lambda Initiative Medium Access Control, as a mechanism to provision bandwidth and guarantee a services framework in flexible wavelength spaced systems. The article describes the implementation of OLIMAC. The OLIMAC solution is shown to work with different protocols showcasing backward compatibility with existing end-user technologies while facilitating voluminous bandwidth to end users. The OLIMAC concept is simulated, and results show the benefit of the proposal as well as how OLIMAC is instructive in the implementation of NGPON2, especially for multirate flexible wavelength-spacing solutions.