1995 BMW 740

Fews days mt car overheated, since them I changed the water pump, radiator expansion tank termostast, hose. But a lot aire ccome every time I start the car. I bleeded all I could. But geting overheated after short runiing. Now the computer showme a new sign, I neverf see before, that said pression oil too low. Stop the engine. So I stop. Next day was very cool I start the car. No sign of oil while was cold, afyer getting hot, the sign came again. Also I chequed oil. No water, but the exaust pipe in the driver side starting to fume, like in the winter time. Any sugestion?