Did you use teflon tape on the sender's threads?
Have you checked continuity of the sender's base to ensure a good ground connection?
Have you supplied power to the wire going to the dash to make sure the bulb and all places in between are good?
Have you cleaned the connection and ensured it being tight on the end of the sender to the wire?
Did you do anything else under the dash or under the hood to perhaps loosen a wire?
Did you check your actual oil level and also pressure with a mechanical gauge?
Do you have a ground strap from the engine block to the body?
Answer the above and it will lead us to helping you.