Member Monte_ExpreSS covered it pretty well with this blurb:
"Under the dash, near the steering column will be a small, green box. There should be 2 or 3 (can't remember) wires coming off of it heading up in the direction of the speedo. On the end of the wiring is a black plastic optic eye. That is your VSS. it bolts to the back of the speedo & is for the purpose of sending a speed signal to the ECM & cruise, if your car has it. It does NOT control the speedo inself, our cars use a cable driven unit. If the speedo itself is not working correctly, the VSS won't get a reading & therefore show a code 24. Find out what the issue with the speedo not reading is, & the code shoudl go away."
Full thread here: VSS (vehicle speed sensor )
When he says "if your car has it", I do believe he's just referring to cruise control. Your car has an ECM and it is expecting a VSS, your own findings prove that.
You talked about how your speedo reading appears to be incorrect. I'd start by fixing that and see if your code 24 problems go away. Your simplest bet might be to swap the entire speedometer, though you'll have to expect to pay over $100 for a used one unless you get really lucky and find one in a junkyard or something. If you end up having to import one from the US, I can tell you for sure that the faceplate will interchange with your existing KPH faceplate, but you will have to carefully remove and replace the indicator needle which is a PITA because then you have to get it recalibrated (I did it with trial and error, took about 10 tries)
Without an O2 sensor you must be getting terrible gas mileage and your car is likely performing very poorly. I haven't studied what happens once you throw the O2 code, I would guess it runs in Open Loop the entire time, no good for your ride in any case!