O is for Overexposure
Overexposure is a photography technique where the exposure triangle is tweaked to allow more light than the average 18% grey to hit the camera sensor. This results in an overexposed image with a lot of detail in the shadows and darker areas of the picture. It can also lead to hot spots in the light areas where the camera has recorded pure white over a group of pixels leading to a 'blown out' area on the picture.
Overexposure is currently a popular trend in lifestyle photography, where photographers expose for the subject with the key light or sun behind the subject, leading to a very light background, and often a high key look overall.