You are ticketed AMS-ORD with a connection at LHR. The only thing that matters is the delay at your final ticketed destination, e.g. ORD and the operating carrier of the flight which causes your delay, e.g. BA.
If your arrival at ORD is 3+ hours, you will be due EUR 300 and if 4+ hours, EUR 600. No way of knowing anything and not worth even thinking about until after your arrival at ORD.