I am using toppik fiber and I want apply oil on my head can I do this yes or no?

Oil and Toppik does't sound like a normal thing to do. It is best to contact your doctor about post operative care and instructions since each doctor has a different regimen.

Hi Qureshi_bhai, thank you for your
question on using oil and Toppik thickening fibers. You could put oil on your
head and you could put Toppik in your hair once your doctor has given
permission to do so. This is usually after your scalp has had time to
sufficiently heal. Toppik when used properly is a very convincing concealer,
however I am not sure if oils on the scalp would diminish this. I would
practice with it when you are not going out. Once you feel confident in it's
proper application then why not use it. I am surprised after two sessions you
feel this is something you feel you need to continue. 

You can try almost anything you want to try after the hair transplant is fully healed. Best, of course, to see your doctor and get some good, focused advice.