1972 Chevrolet Cheyenne 72 Chev 1 2 Ton Pu Turn Key On Will Show Oil Light

72 Chev Cheyenne 1/2 ton pu. Turn key on and will show oil light but motor won't turn. Can get under pickup and start it with a screw driver. Replaced Cylenoid on starter and but new ignition swich in. Still will not start with key. There is no safety switch on this truck can start it in drive. What could be the problem