Phonon–magnon coupling has been extensively studied in ferromagnets, whereas its microscopic mechanisms in antiferromagnets remain less understood. Here, we combine first-principles calculations with spin–lattice dynamics to investigate phonon–magnon interactions and their impact on thermal transport in antiferromagnetic NiO and MnO, which are put in contrast to the ferromagnetic EuO with an identical crystal structure. Green–Kubo and spectral energy density analyses show that spin dynamics reduce phonon thermal conductivity and lifetimes, with the strongest effects in MnO. Comparison of the excitation spectra indicates that spin-induced phonon scattering depends sensitively on the relative phonon and magnon energy scales, while the contrasting behavior of MnO and EuO further highlights the role of magnetic order. A simplified phase-space model further illustrates that, for comparable phonon and magnon energy scales, the antiferromagnetic case can support a larger phonon–magnon scattering phase space than the ferromagnetic case, suggesting a greater number of kinematically allowed scattering channels. These results identify excitation energy scales and magnetic order as key factors governing phonon–magnon scattering in magnetic materials.