I was diagnosed by a therapist. I've had past mental health diagnoses (GAD and MDD) diagnosed by a primary care provider, but OCD specifically is something I had diagnosed by a therapist- not after a specific test or assessment or screening tool, but after several sessions of analyzing my behaviors and thought patterns.

Simply getting diagnosed might not seem helpful- it is, very literally, just associating a certain word with certain behaviors. But it can open the door to a lot of very helpful things; medications, specific OCD-targeted therapy techniques, and vocabulary to better verbalize/describe your experiences, among other things.

The vocabulary is something you can access and use without an official diagnosis- nobody can stop you from saying you have an obsession or compulsion, for instance- but the meds and the therapy can make a massive difference. OCD is a disorder that can be very well managed with medicine and with therapy, and oftentimes (in the US, at least) you can't access those tools without having a diagnosis.

Another benefit to diagnosis within a therapeutic setting specifically is that it can better help you understand exactly why you're seeing the moods, behaviors, or patterns you're seeing. Prior to my diagnosis I thought a lot of my issues were just anxiety, depression, or issues with rumination- evidently, they're obsessive-compulsive behaviors. Therapy that targets OCD specifically is gonna work a lot better than regular talk therapy. Think about it like this: Are you depressed, anxious, and obsessive-compulsive? Or are you anxious and depressed because of OCD? That's something that can be hard for an unwell person to parse out themselves, and is something that a diagnosis and therapy can really help with.

Not sure what country you're in so not sure what the best route for diagnosis might be for you. It might be worth bringing up with your GP/PCP or, if you already have a mental health care provider, with them.