The CMB presents a unique probe of dark matter physics. Ultra-light axions of mass around 1e-22 eV are a promising dark matter candidate well motivated by high energy physics. The Simons Observatory (SO) is a new cosmic microwave background (CMB) experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. I will highlight some of the cosmology forecasts for SO science, and focus on the constraints for ultralight axions.