Do Orthodox use the plain cross?

Yes, and they also use many other crosses, such as the Orthodox cross (which has three bars), the Patriarchal cross (which has two bars), the Greek cross (which has equal sides), the Latin cross (which has a larger bottom bar), the Celtic cross (which has a circle in the middle), and various other crosses. The Latin cross is commonly used in the West, and is also known as the plain cross. The crosses most commonly seen in Orthodox churches are the Orthodox cross and the plain cross.