Ex: She was one of the teachers who was selected.
Who refers back to the noun phrase one of the teachers, the head of which is the singular noun one, which is why the verb is singular. You can test this:
Ex: One (of the teachers) was selected. Additionally, relative pronouns (e.g., who) refer back to the closest noun. In our example, the plural noun teachers is the closest noun from a linear perspective; i.e., it's next to who, but syntax doesn't work in linear terms. The prepositional phrase of the teachers is embedded or hidden inside ([...]) the noun phrase one of the teachers,
... [one [of the teachers]] who was ...Structurally, who is closer to the noun one ([...]) than it is to the noun teachers, and the reason the verb is singular was. The verb cannot be plural as who cannot see inside the prepositional phrase ([...]).