Methods and apparatus for a diagnostic ring oscillator (RO) circuit for DC and transient characterization. The RO circuit includes a plurality of symmetrical stages coupled via a feedback signal line and forming an inverter chain, where each stage includes a CMOS inverter comprising a pair of pMOS and nMOS transistors coupled between power-gating transistors respectively coupled to a positive voltage source and ground. An output of a CMOS inverter for the stage is coupled to an input for the CMOS inverter of a next stage. The first stage is an enable stage configured to set the inverter chain into a defined logic state, followed by multiple pre-stageDUT stages. The output of the last stage is feed back to the input of the enable stage to form a feedback signal. The RO circuit can operate in multiple modes including an AC mode, a DC mode, and a hybrid mode.