The help centre, the Essentials Guide and the North America manual.
Let the oven go completely cold.
Outside: wipe soot off with a dry microfibre cloth or paper towel. For tougher marks use warm soapy water, then dry it at once. No abrasive or harsh cleaners.
Heat tint on the steel brow: a stainless steel cleaner on the outside only. Ooni names Bar Keepers Friend Power Cream and Weiman.
Inside: dry paper towel, and the Ooni brush for stubborn bits. No water inside.
Legs and other parts: a non-abrasive sponge (the manual).
Near the sea, rinse salt off the outside and dry it (help centre).