Optical isolators, devices that only allow unidirectional light propagation, constitute an essential building block for photonic integrated circuits. For near-infrared communications wavelengths, most current isolator designs rely on the incorporation of magneto-optical (MO) materials to break time-reversal symmetry, such as iron garnets or magnetically substituted semiconductors. MO garnets form the backbone of traditional bulk isolators, but suffer from large lattice and thermal mismatch with common semiconductor substrates, which has significantly impeded their integration into on-chip optical isolators. Materials innovations over the past few years have overcome these barriers and enabled monolithic deposition of MO oxide thin films on silicon using techniques such as pulsed laser deposition and magnetron sputtering. On-chip optical isolator devices with polarization diversity in the telecommunication band have been demonstrated based on these materials. This article reviews the latest technological breakthroughs in MO oxide material growth as well as device design and integration strategies toward practical implementation of on-chip optical isolation.