OSCAR stands for "Only Style Classes, AlRight?". It's a CSS methodology that is designed to be very simple and lead into other mentalities such as OOCSS, SMACSS or ITCSS. The idea is to take the most basic parts of the other methodologies and create some simple rules that can be quickly taught to a new-comer without weighing them down with complexities. This is the starting point for better CSS.
No IDs, no elements. Only classes. You can have up to 1 class, 1 pseudo-element and 1 state or pseudo-class.