There are several ways of going about doing this, and very few of them actually use the CP instruction, since that only works between two one byte values, whereas an OP "register" is a memory construct and is not directly supported by the CPU (meaning you have to do this in several steps to simulate support, which is what ASM is really about)

1. Compare the two OP registers directly by first comparing the exponent. If one is greater than the other, you can stop already. If not, then compare each byte afterwards until something happens. What that something is... is up to you.
ld DE,ADDRESSOFFIRSTOPREG+
 ld HL,ADDRESSOFSECONDOPREG+
 ld b,8
Loop:
 ld a,(de)
 cp (hl)
 jr nz,SomethingHappened
 djnz Loop
 ret
SomethingHappened:
;carry=(DE)<(HL)
;your code here
 ret
2. If your numbers are integers, use the romcall ConvOP1 to convert to DE, swap out to HL, preserve it, move second value to OP1 and do the call again, then use the or a \ sbc hl,de sequence, since that works best as a two-byte CP instruction.
bcall(_convop1)
 push de
   ;move other value to Op1
   bcall(_convop1)
 pop hl
 or a
 sbc hl,de
 ;carry if second value was greater than first convOp1 value
 ;insert your code here
 ret

3. Get the OS to do the comparison for you BASIC-style, except you're directly calling a parser function. Today, it's _BinOPExec (for two-argument functions), where the first argument is in Op1 and the other pushed to the FPS. You then load register A with the function to execute before using the romcall.
;arguments are assumed to be in place already
 ld a,FUNCTION
 bcall(_BinOPExec)
; Op1 will contain the result of the operation. If you use something
; like OPEq, OPGt, or OPLt, you'll get 0 for false or 1 for true.
; You can then use ConvOP1 to reduce that to register A then compare it.

As with all unfamiliar romcalls, you must read the documentation for further information.

Hey Ephraim.  I know you linked to the Cemetech topic, but I would suggest editing your first post and give a brief description of what WizardC8 is. It'll be a lot nicer and neater than simply linking to another forum topic,(which people may not want to check) and will have all the info available here.