The OC can be strong by having unwavering moral principles. For example, if Draco tries to bully someone, the OC stands up to him firmly, not afraid of his family name or his Slytherin cronies. Also, the OC can be highly intelligent in a different way than Draco. Maybe she's excellent at non - verbal magic or has a deep knowledge of ancient runes that Draco respects and is a bit envious of. This makes her a force to be reckoned with in their interactions.
The OC can be strong in the face of Draco by being extremely empathetic. While Draco may be cold and calculating at times, the OC can understand the insecurities and pressures that he's under because of his family. However, she doesn't let this understanding stop her from calling him out on his wrongdoings. She uses her empathy as a tool to try and change him for the better. This makes her a complex and strong character as she's not just reacting defensively to Draco, but also trying to have a positive impact on him.