Depends on the context. An "op code" is one of the processor's instructions. Naturally you can write it in binary, and then I suppose it's a binary op code. It would often be listed that way in the list of assembler instructions at the back of a tutorial book. It actually makes quite a lot of sense to list x86 opcodes in binary because large chunks of them follow a set pattern. For instance the registers eax, ebx, ecx, edx (or associated ax, bx, cx, dx) are encoded by 2 bits of binary.