How did you flush the heater core or did you flush the whole system?

 

Just the heater core. After further driving, the film has come back. So not sure what the issue is.
I used a basic flush kit from AZ, ran water through it, reversed the inlet/outlet and ran water through it again. It worked for about a week. Then the smell and film returned.

The O2 sensor fix worked though.

 

You will likely have to replace the heater core or a related hose by the sounds of it. Once they start leaking even just a little it needs to be replaced, it will only get worse. And I wouldn't even consider that stop leak snake oil garbage they sell that will supposedly "fix" the leak, it won't but it will gunk up your cooling system. Usually a pressure check will expose a weak or leaky heater core as it will start leaking much worse quickly. A new heater core is mostly labor and a lot of it.