BMX = American
BMX adapted old American standards for parts, so if you encounter a part that looks like it will fit and it's labelled "BMX," it will work for your bike. OPC chainrings come in 1/2" (for single speed chain) or 3/32" (5-6 speed chain) sizes. 3/32" will work with either chain size.

If you can't find an OPC chainring you like, you can buy a spider that will let you use regular chainrings (IIRC, Porkchop BMX has them on clearance cheap) or you can get an OPC to 3-piece adapter from Truvative and run square taper cranks.