Obstacle Course Racing — an event requiring an athlete to run through a variety of different obstacles.
Offroad Boarding — another name for Mountainboarding
Off-Road Racing — many motor sports have competitions 'off-road', meaning on rough terrain.
Oil Wrestling — a form of wrestling called Yağlı Güreş in Turkey, is where the wrestlers cover themselves in oil.
Oină – a Romanian traditional sport, similar in many ways to Baseball and Lapta played outdoors by two teams of 11 players, taking turns either batting or catching.
Okinawan Kobudō — weapon systems of Okinawan Martial Arts, also known as Ryūkyū Kobujutsu.
One Day International (ODI) — a form of Cricket played in a single day, usually 50 overs per team. An even shorter version is the Twenty20.
open water (ocean) swimmingopen water (ocean) swimming
One-Pocket — a version of Pocket Billiards where the objective is to pocket all the object balls into a single pocket.
Open Water Swimming — competitive swimming races conducted over various distances (10km at the Olympics) held in rivers, lakes and oceans.
Orienteering — participants find their way to various checkpoints across rough country with the aid of a map and compass, the winner being the one with the lowest elapsed time.
Outdoor Handball — another name for Field Handball.
Outrigger Canoeing — racing using a type of canoe featuring one or more lateral support floats known as outriggers, which are fastened to one or both sides of the main hull.
Over-the-line – a bat-and-ball sport related to baseball and softball with just 3 people per team. Unlike the other sports, the batter and pitcher are on the same team.
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More answers
Here are a few I have found: Off-Road Multisport, Offroad
Vehicles, Offshore Boat Racing, Olympic Lifting, Open Water
Swimming, Orienteering, Outrigger, Off-Road Skating, Outdoor
Sports, Outdoor Camping, Open Fishing.