Taken from the OCD Workbook: Your Guide to Breaking Free of OCD.
Intrusive thought --> Catastrophic appraisal of the thought --> Increased anxiety and worry --> Overcontrol strategies which include Vigilence (Could I really do it? and Covert rituals (testing) --> temporary anxiety reduction --> The cycle starts over again.
"Primarily obsessional OCD begins with an intrusive, distressing thought that the person appraises in a particularly negative way. In the person's mind, the thought is experienced as having the same reality or importance as an action. As a result, the person attempts to avoid or suppress the thought. This leads to various overcontrol strategies to reduce anxiety, including mental rituals. While these strategies may reduce discomfort, the effect is only temporary, and then the cycle begins again."