I retrieved codes P0147( Oxygen Sensor-Heater Perfromance Sensor Bank 1 / Sensor 3) and B2118 ( Front Vertical Sensor) from my 1999 Cadillac DeElegannce ( 36,000 miles)
Can someone explain these codes to me, and tell me if replacing the oxygen sensor will solve the P0147 code?

Most likely replacing the B1S3 O2 sensor will solve the P0147 code. That is the aft-cat sensor.

Thanks again Ranger.
I see your located in Woodstock,I'm Lake Zurich.
Have a brother at Ford dealership in Woodstock.

Practically neighbors. I had a neighbor that used to work at that Ford dealer.

What do you mean aft-cat sensor?
Checked a oxygen senson website and they show four possible sensors.

Correct.
Bank 1 Sensor 1 = right bank (firewall) exhaust manifold
Bank 1 Sensor 2 = pre-cat
Bank 1 Sensor 3 = aft-cat
Bank 2 Sensor 1 = leftbank (front) exhaust manifold
cat = catalitic converter

Got It! THANKS.