Hi @kefir135
I'm battling with this myself at the moment but I am making some progress.
As far as I know there still isn't an option to create an OAuth client in the dashboard, however it can be be done quite easily via curl and only needs to be done once per app so it's not a biggy.
If understand correctly, you're still trying to create a customer via OAuth client? If so, take a look at my thread here, with the help of the Particle team I've just managed to submit an HTML form to create a customer for my organisation using my client_id and then redirect them to my setup page.
Still a bit of work to go but getting there!