  O-stars are known to experience a wide range of variability mechanisms
originating at both their surface and their near-core regions. Characterization
and understanding of this variability and its potential causes are integral for
evolutionary calculations. We use a new extensive high-resolution spectroscopic
data set to characterize the variability observed in both the spectroscopic and
space-based photometric observations of the O+B eclipsing binary HD~165246. We
present an updated atmospheric and binary solution for the primary component,
involving a high level of microturbulence ($13_{-1.3}^{+1.0}\,$km\,s$^{-1}$)
and a mass of $M_1=23.7_{-1.4}^{+1.1}$~M$_{\odot}$, placing it in a sparsely
explored region of the Hertzsprung-Russell diagram. Furthermore, we deduce a
rotational frequency of $0.690\pm 0.003\,$d$^{-1}$ from the combined
photometric and line-profile variability, implying that the primary rotates at
40\% of its critical Keplerian rotation rate. We discuss the potential
explanations for the overall variability observed in this massive binary, and
discuss its evolutionary context.
