My tank temp will go from 78 to 80 when I turn my light on. It takes 10-12 hours for this to happen. Is this 2 degree change okay for my Oscars and Dempseys? I am using a 4 foot light strip with 1 bulb.

This is quite normal, your lights will generate heat which in turn will raise the temperature of the water. I shouldn't worry, in the wilds, temperature of the water will be changing all the time, at night, and when it rains. In an aquarium, as long as you don't have radical changes in temperature in a very short period, your fish really won't notice any difference and will not come to any harm.

I agree with OFL, this temperature change is nothing to worry about

i concur with the guys...however, abrupt temp changes is similar to
abrupt pH changes that could stress your fish and will lower their
immune system resulting to sickness.