  Widely-tunable coherent sources are desirable in nanophotonics for a
multitude of applications ranging from communications to sensing. The
mid-infrared spectral region (wavelengths beyond 2 $\mu$m) is particularly
important for applications relying on molecular spectroscopy. Among tunable
sources, optical parametric oscillators typically offer some of the broadest
tuning ranges; however, their implementations in nanophotonics have been
limited to narrow tuning ranges and only at visible and near-infrared
wavelengths. Here, we surpass these limits in dispersion-engineered
periodically-poled lithium niobate nanophotonics and demonstrate ultra-widely
tunable optical parametric oscillators. With a pump wavelength near 1 $\mu$m,
we generate output wavelengths tunable from 1.53 $\mu$m to 3.25 $\mu$m in a
single chip with output powers as high as tens of milliwatts. Our results
represent the first octave-spanning tunable source in nanophotonics extending
into the mid-infrared which can be useful for numerous integrated photonic
applications.
