Has anyone ever wondered this? They trust him enough to tell him that the children are the Jewels, and he helps them plan their escape to the Ice Prairies, and he seems to know all their secrets, so they must be very good friends indeed! But he's not from Anniera. Is he an old friend of Podo's from over 50 years ago? He doesn't seem like the kind of person Podo would have been friends with back then. Did Nia seek him out because she needed help teaching the children their THAGS? Book 1 does say that Oskar was the one who taught Janner to love books and gave him his first journal. Maybe Nia and Podo figured out that Oskar was involved with the resistance and that made them start to trust him?
What do you think? Try to avoid any major spoilers beyond book 1!
Being the only bookstore in Glipwood would mean the children would frequent there often for schooling and recreational activity. I think Oskar happened to find common ground with Janner and took him under his wing and the rest of the family followed