Secrets

One of the Secrets in OvO in 1.2.3 plus is that when you're on the menu, click on credits, then click on the spinning sand timer, it will unlock all levels in the level tab in the menu.

On 1.4.1 to 1.4.4, this will also unlock all Skins you get for beating certain categories. This includes OvO+, Electrical, Astronaut, Pulse, MaterWelon, and even The Fallen.