AbstractUnderstanding the mechanism of O-star formation is one of the most important current issues in astrophysics. Also an issue of keen interest is how O stars affect their surroundings and trigger secondary star formation. An ${\rm H\,\small{II}}$ region RCW 79 is one of the typical Spitzer bubbles alongside RCW 120. New observations of CO J = 1–0 emission with Mopra and NANTEN2 revealed that molecular clouds are associated with RCW 79 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 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 over a timescale of ∼3 Myr. RCW 79 has a larger size by a factor of 30 in the projected area than RCW 120 with a single O star, and the large size favored formation of the 12 O stars due to the greater accumulated gas in the collisional shock compression.