From what I could gather is that the thermometer is designed for an open group head. Is there a way to modify Erics thermometer adaptor for the Oscar or is there another way of measuring the group head temp during a shot?

Various people have put thermocouples and RTDs just above the coffee.

...is there another way of measuring the group head temp during a shot?

You can measure the group head temp during a shot simply by using a thermocouple attached to the group head with tape.
If you want to measure the Brew water temp during a shot, simply place the thermocouple on top of the puck before you attach the portafilter. the t/c wire will just bed itself into the group seal.
T/c 's for use with multimeters are cheap.

You can use simple thermocouple above shower screen:
Nuova Simonelli Oscar mods