p0300 = random cylinder misfire
p0307 = misfire cylinder #7
It's not going to be an O2 sensor. The 6.0L has a "coil near plug" system in that the individual coils are mounted on the valve covers with a short wire to the plug.
As daredevil said swap the coils with an adjacent cylinder and see if the code moves to that cylinder, if it doesn't swap the wire, the plug and lastly the injector. If it never changes what cylinder it sets the misfire code for then you may have some slight coolant intrusion into that cylinder. But that would be the last thing I would worry about.