I have an oblique tear of the posterior horn and body of the medial meniscus extending to the inferior articular surface. what is the treatment?

Knee scope: If you are having pain, swelling and catching, then the only reasonable option would be arthroscopic knee surgery. If your symptoms completely resolve with just anti-inflammatories with no catching symptoms, then it might be a chronic degenerative tear, and you can choose to live with it as is.

Knee pain: Depending on your duration of symptoms you can at least start off with physical therapy, a knee sleeve, and if there is arthritis present consider a course of hyaluronic acid injections. If all fails then a discussion re: arthroscopic surgery can proceed.