In obsessive compulsive disorder or OCD obsessions are the intrusive and repetitive images and thoughts which are extremely distressing to the individual and compulsions are the act the individual ritualistically performs to reduce the distress. The compulsion here is a direct result of the obsession.
The common types of OCD such as cleaning/washing, aggressive type or the symmetry type can be easily detected. They all have an obvious compulsive behaviour the individual engages often engages in. But there is type of OCD that is very hard to notice, they are commonly known as “pure-O” or pure obsessions. Here the individual only tends to have obsessions.
Many a times in pure-O the individual may belief for years that they do not have any obvious compulsive behaviour and in some cases it can be true. But in most cases the individual does have a compulsion, mental compulsion acts. In this mental act the individual pays deep attention to the obsessive thought or image and starts questioning them over and over and try to find possible solutions. Read more about OCD Ruminations here