The performance of transformers in the presence of oscillatory transients is addressed in this guide. Oscillatory transients are typically produced by the interaction of the switching device, transformer, load, and system. This guide describes operating conditions that may produce oscillatory switching transient voltages damaging to the transformer insulation system. It discusses the electrical characteristics of the source, switching device, transformer, load and the nature of their transient interaction. It discusses several mitigation methods. Two generic examples are included. This guide focuses on mechanical switching devices and does not address semiconductor switching devices.