  We present a variability study of 4646 massive stars in the Small Magellanic
Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010)
using the light curves from the OGLE-III database. The goal is to exploit the
time domain information available through OGLE-III to gain insight into the
processes that govern the evolution of massive stars. This variability survey
of massive stars with known spectral types is larger than any previous survey
by a factor of 7. We find that 60% of our sample (2766 stars) show no
significant variability and 40% (1880 stars) exhibit variability distributed as
follows: 807 stars display low-amplitude stochastic variability with
fluctuations in I-band of up to 0.05 mag, 443 stars present irregular
variability of higher amplitude (76% of these are reported as variables for the
first time), 205 are eclipsing binaries (including 101 newly discovered
systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188
stars exhibit short-term sinusoidal periodicity (P < 3 days) making them
candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61
periodic stars exhibit longer periods. We demonstrate the wealth of information
provided in the time domain, by doubling the number of known massive eclipsing
binary systems and identifying 189 new candidate early-type Be and 20 Oe stars
in the SMC. In addition, we find that ~80% of Be stars are photometrically
variable in the OGLE-III time domain and provide evidence that short-term
pulsating stars with additional photometric variability are rotating close to
their break-up velocity.
