  We present results of a high angular resolution survey of massive OB stars in
the Cygnus OB2 association that we conducted with the NIRI camera and ALTAIR
adaptive optics system of the Gemini North telescope. We observed 74 O- and
early B-type stars in Cyg OB2 in the $JHK$ infrared bands in order to detect
binary and multiple companions. The observations are sensitive to
equal-brightness pairs at separations as small as 0.08 \arcsec, and
progressively fainter companions are detectable out to $\Delta$ K = 9 mag at a
separation of 2 arcsec. This faint contrast limit due to readnoise continues
out to 10 arcsec near the edge of the detector. We assigned a simple
probability of chance alignment to each companion based upon its separation and
magnitude difference from the central target star and upon areal star counts
for the general star field of Cyg OB2. Companion stars with a field membership
probability of less than 1% are assumed to be physical companions. This
assessment indicates that 47% of the targets have at least one resolved
companion that is probably gravitationally bound. Including known spectroscopic
binaries, our sample includes 27 binary, 12 triple, and 9 systems with four or
more components. These results confirm studies of high mass stars in other
environments that find that massive stars are born with a high multiplicity
fraction. The results are important for the placement of the stars in the H-R
diagram, the interpretation of their spectroscopic analyses, and for future
mass determinations through measurement of orbital motion.
