<p>Optical parametric oscillators (OPOs) are widely utilized in photonics as classical and quantum light sources. Conventional OPOs produce co-propagating signal and idler waves that can be either degenerately or non-degenerately phase matched. This configuration, however, renders their frequencies highly sensitive to external disturbances. Here we demonstrate a degeneracy-locked OPO achieved through backward phase matching in a submicrometre periodically poled thin-film lithium niobate microresonator. Although backward phase matching establishes frequency degeneracy of the signal and idler, the backscattering in the waveguide further ensures phase locking between them. Their interplay permits the locking of the OPO’s degeneracy over a broad parameter space, resulting in deterministic degenerate OPO initiation and robust operation against both pump detuning and temperature fluctuations. This work, thus, provides a new approach for synchronized operations in nonlinear photonics and extends the functionality of optical parametric oscillators. With its potential for large-scale integration, it provides a chip-based platform for advanced applications, such as squeezed light generation, coherent optical computing and investigations of complex nonlinear phenomena.</p>