If you receive a Notice of Opposition from a third party, all may not be lost. Oppositions may sometimes be successfully resolved or challenged.
In general, we would recommend seeking out the counsel of a trademark attorney who can help with proper conduct in this situation.
There are multiple approaches to resolving opposition, depending on whether it has merit, the nature of similarity between the two brands, and the other party's willingness to reach an amicable solution. Your attorney will be able to assess the situation and recommend the best course of action, but if you want to see the most common resolution tactics, you can read our article What happens if someone opposes my trademark application?