  Mo$_8$O$_{23}$ is a low-dimensional chemically robust transition metal oxide
coming from a prospective family of functional materials, MoO$_{3-x}$, ranging
from a wide gap insulator $(x=0)$ to a metal $(x=1)$. The large number of
stoichometric compounds with intermediate $x$ have widely different properties.
In Mo$_8$O$_{23}$, an unusual charge density wave transition has been suggested
to occur above room temperature, but its low temperature behaviour is
particularly enigmatic. We present a comprehensive experimental study of the
electronic structure associated with various ordering phenomena in this
compound, complemented by theory. Density-functional theory (DFT) calculations
reveal a cross-over from a semi-metal with vanishing band overlap to narrow-gap
semiconductor behaviour with decreasing temperature. A buried Dirac crossing at
the zone boundary is confirmed by angle-resolved photoemission spectroscopy
(ARPES). Tunnelling spectroscopy (STS) reveals a gradual gap opening
corresponding to a metal-to-insulator transition at 343 K in resistivity,
consistent with CDW formation and DFT results, but with large non-thermal
smearing of the spectra implying strong carrier scattering. At low
temperatures, the CDW picture is negated by the observation of a metallic Hall
contribution, a non-trivial gap structure in STS below $\sim 170$ K and ARPES
spectra, that together represent evidence for the onset of the correlated state
at $70$ K and the rapid increase of gap size below $\sim 30$ K. The intricate
interplay between electronic correlations and the presence of multiple narrow
bands near the Fermi level set the stage for metastability and suggest
suitability for memristor applications.
