Abstract: I describe bosonic (scalar dark matter, electromagnetic and gravitational) wave mixing in curved spacetime using novel gravitational wave effective field theory techniques. Curved spacetime adds a new length scale, the Schwarzschild radius, which significantly alters the oscillation probabilities in comparison to the standard flat spacetime computations. The curved spacetime effects are analogous to the Mikheyev-Smirnov-Wolfenstein (MSW) effect for neutrinos and are ``frozen-in," as the bosonic wave propagates away from the compact object. Although I will consider axions and axion-like particles (ALPs), our computations are model-independent and applicable to any bosonic dark matter candidate. In particular, I describe how the mixing often occurs via the energy-momentum tensor, instead of the phenomenological couplings such as the axion-photon coupling. Time permitting, I describe the associated oscillation probabilities and discuss some of the observational consequences of the mixing, including the energy and polarization of the exiting electromagnetic and gravitational waves.