  Understanding the mechanism of O star formation is one of the most important
issues in current astrophysics. It is also an issue of keen interest how O
stars affect their surroundings and trigger secondary star formation. An
H\,\emissiontype{II} region RCW79 is one of the typical Spitzer bubbles
alongside of RCW120. New observations of CO $J=$ 1--0 emission with Mopra and
NANTEN2 revealed that molecular clouds are associated with RCW79 in four
velocity components over a velocity range of 20 km s$^{-1}$. We hypothesize
that two of the clouds collided with each other and the collision triggered the
formation of 12 O stars inside of the bubble and the formation of 54 low mass
young stellar objects along the bubble wall. The collision is supported by
observational signatures of bridges connecting different velocity components in
the colliding clouds. The whole collision process happened in a timescale of
$\sim$1 Myr. RCW79 has a larger size by a factor of 30 in the projected area
than RCW120 with a single O star, and the large size favored formation of the
12 O stars due to the larger accumulated gas in the collisional shock
compression.
