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Optomechanical sensors enable exquisitely sensitive force measurements, with emerging applications across quantum technologies, standards, fundamental science, and engineering. Magnetometry is among the most promising applications, where chip-scale optomechanical sensors offer high sensitivity without the cryogenics or magnetic shielding required by competing technologies. However, a lack of compatibility with integrated photonics and electronics has posed a major barrier. Here, we introduce silicon-on-insulator optomechanical magnetometers to address this barrier. A new post-release lithography process enables high-quality metallization of released mechanical structures, overcoming the incompatibility between silicon-on-insulator fabrication and functional magnetic films. This allows us to employ photonic-crystal cavities that enhance motion-to-optical signal transduction by over an order of magnitude. The resulting devices achieve magnetic field sensitivity of 800pT/Hz, three orders of magnitude beyond previous waveguide-integrated designs. The advance report we provide, leads to a path toward high-performance, room temperature, and chip-integrated magnetometers for applications ranging from biomedical imaging and navigation to resource exploration.