By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate τ of the asymmetric-stretch vibration (ν3=1136cm-1) of oxygen (Oi) in silicon changes by a factor of ∼2.5. These data establish that ν3 decays by creating one ν1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (ν3-ν1) is less than the maximum frequency νm of the host lattice, as for Si28-O16-Si28 in natural silicon, then it is emitted as one lattice mode, and τ depends on the density of one-phonon states at ν3-ν1. If (ν3-ν1)>νm, as for O16 in single-isotope Si30 silicon, two lattice modes are created in addition to ν1, increasing τ. Prediction of τ for a particular defect clearly requires a detailed knowledge of that defect.