[adj] pertaining to or concerning the outdoors or outdoor activities; "outdoor education is the area of teacher training concerned with training for outdoor activities"
OUTDOOR is a 7 letter word that starts with O.
Webster's 1913 Dictionary
Definition:
\Out"door`\, a. [For out of door.]
Being, or done, in the open air; being or done outside of
certain buildings, as poorhouses, hospitals, etc.; as,
outdoor exercise; outdoor relief; outdoor patients.
WordNet Dictionary