Nearly all men will be able to have an orgasm following prostate cancer treatment. This is true even in most cases of erectile dysfunction.
The nerves and blood vessels that are most commonly damaged by prostate cancer treatments are very important for achieving an erection, but are not very important for achieving orgasm. Therefore, though there is the occasional man who may be unable to achieve orgasm after prostate cancer treatment, the vast majority will still have this sexual function.
It's important to note that the prostate is the gland that produces most of the fluid that makes up semen. Thus, once the prostate has been removed by surgery or destroyed by radiation, a man's orgasm will be nearly or completely dry.
If you are experiencing difficulties with achieving orgasm after prostate cancer treatment, be sure to talk to your doctor.