OB associations are historically thought to be the primary driver of triggered
OB star formation since 1970's (Elmegreen and Lada 1977) .
It is not a usual idea that M42 and M43, the best known HII regions in Orion,
were formed by external triggering of cloud cloud collision instead of the
radiative/wind triggering driven by OB stars. I present a recent reanalysis of
high resolution CO images, and present evidence for cloud cloud collision in
M42 and M43. This collision is possibly the real driver of the formation of
the OB stars ionizing M42 and M43. I discuss implications of these results in
the context of star formation history of the rich cluster environment in the
nearest high mass star formation.
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