  We present a database of $BVRI$ time-series photometry of the Orion Nebula
Cluster obtained with two ground-based telescopes at different longitudes to
provide simultaneous coverage with the 13-d Chandra observation of the cluster.
The resulting database of simultaneous optical and X-ray light curves for some
800 pre--main-sequence (PMS) stars represents, by a factor of hundreds, the
largest synoptic, multi--wavelength-regime, time-series study of young stars to
date. This database will permit detailed analyses of the relationship between
optical and X-ray variability among a statistically significant ensemble of PMS
stars, with the goal of elucidating the origins of PMS X-ray production. In
this first paper, we present the optical observations, describe the combined
X-ray/optical database, and perform an analysis of time-correlated variability
in the optical and X-ray light curves. We identify 40 stars (representing 5% of
our study sample) with possible time-correlated optical and X-ray variability.
Examples of both positive and negative time-correlations are found, possibly
representing X-ray flares and persistent coronal features associated with both
cool and hot surface spots (i.e. magnetically active regions and accretion
shocks). We also find two possible examples of "white-light" flares coincident
with X-ray flares; these may correspond to the impulsive heating phase in
solar-analog flares. However, though interesting, these represent unusual
cases. More generally, we find very little evidence to suggest a direct causal
link between the sources of optical and X-ray variability in PMS stars. The
conclusion that accretion is a primary driver of X-ray production in PMS stars
is not supported by our findings.
