Hi Peter,
Would format of subject of your sales opportunity looks like below?
e.g: Interested in a large quantity of "X"

If so, we should firstly know that segment could only target at Contact entity,
then to define such segment, please follow following steps:
1. Navigate to Settings > Marketing data configuration

2. Select Opportunity entity then Publish Changes, it will make the entity available in query block dropdown list.

3. After half an hour, create a segment:
Opportunity Topic contains "X",
select relationship Contact (Opportunity -> Contact (Contact (parentcontactid)))

Go live the segment to check retrieved contacts:

Regards,
Clofly