  We report dynamical mass measurements of the individual stars in the most
luminous and massive stellar member of the nearby Ophiuchus star-forming
region, the young tight binary system S1. We combine 28 archival datasets with
seven recent, proprietary VLBA observations obtained as part of the
\textit{Dynamical Masses of Young Stellar Multiple Systems with the VLBA}
project (DYNAMO--VLBA), to constrain the astrometric and orbital parameters of
the system, and recover high accuracy dynamical masses. The primary component,
S1A, is found to have a mass of 4.11$\pm$0.10~M$_\odot$, significantly less
than the typical value, $\sim$~6~M$_\odot$ previously reported in the
literature. We show that the spectral energy distribution of S1A can be
reproduced by a reddened blackbody with a temperature between roughly 14,000~K
and 17,000~K. According to evolutionary models, this temperature range
corresponds to stellar masses between 4~M$_\odot$ and 6~M$_\odot$ so the SED is
not a priori inconsistent with the dynamical mass of S1A. The luminosity of S1
derived from SED-fitting, however, is only consistent with models for stellar
masses above 5~M$_\odot$. Thus, we cannot reconcile the evolutionary models
with the dynamical mass measurement of S1A: the models consistent with the
location of S1A in the HR diagram correspond to masses at least 25\% higher
than the dynamical mass. For the secondary component, S1B, a mass of
0.831~$\pm$~0.014~M$_\odot $ is determined, consistent with a low-mass young
star. While the radio flux of S1A remains roughly constant throughout the
orbit, the flux of S1B is found to be higher near the apastron.
