Can long-term ocd cause brain cancer?

No,: it won't cause brain cancer.

No: It's terrible to be trapped in your own brain with thoughts like this one. See about both medication and cognitive-behavioral therapy. You're almost an adult. OCD is very common and manageable nowadays.

No, unless there: already is cancer on-board causing the OCD. This is unlikely because you have long-term OCD and the cancer would have grown over time, causing other symptoms. This sounds like a job for a psychologist. CBT is very helpful these days, as are some of the newer meds (that's a job for a psychiatrist). Here's a reference on anxiety: http://www.psychologyproductsandservices.com/page2.html