for some mental disorders yes it is as simple as your medical doctor listening to your symptoms and then saying they believe you have depression and OCD. Some mental disorders dont need long diagnosis process and some do..

for others it isnt so simple, and they need lots of physicals, medical and mental tests..

if you think your doctors diagnosis is not right you can ask him for a referal for a psychiatrist who will do a more lengthy testing process.

my medical doctor diagnosed my PTSD, Depression and Anxiety and depersonalization disorder through knowing me since childhood and listening to my symptoms.

psychiatrists diagnosed my bipolar disorder, DID through a legnthy diagnostic process.

my doctor told me I have depression by looking at me (I was not looking too good back in those depression filled days) and saying - "well looks like the depression bug has bit you big time how about we try some antidepressants and see how you do on them. they should also help with the PTSD too (I had told him about my night terrors and dreams of my abuser breaking out of prison to kill me), here's the name of someone you can go talk with. (therapy)"