You didn't mention your car's year - but till partway through '87 many 240s had a self-destructing wire harnesses.

I'd find that oil sender wire at the firewall connector as described by PLT. Tap into it there. Ground it (use caution as when grounding anything) to verify you have the right one; if the oil light on dash lights up it's the one.

Consider running a fresh lead from the firewall connector along the fender, to the oil sender. I'd run a test/temporary wire via that route to see if that ends your oil-light indication. A red wire from that connector goes to the alternator - the smaller of the two red wires at the alt. That one also sometimes needs a new dedicated wire. The original curls under the timing belt cover as does the oil sender light.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.