Not very familiar with Oxy but am very familiar with American (I teach in the DC area) and a son is at Tufts. I think both AU and Tufts lean very liberal with a very friendly, collaborative student body. Courses at Tufts will be more rigorous than at AU (and I suspect Oxy) given what I have seen from my son’s courses in a range of disciplines (friends teach at AU, so have a sense of the rigor of the courses there, another friend taught at Oxy).
The AU campus is very nice, in some ways nicer than Tufts’, and DC is a great city. Both AU and Tufts are effectively in the suburbs. But access to downtown is quicker at AU. That said, Davis Square (an easy walk from Tufts) is more lively than the n’hoods near AU (including Tenleytown).
For Poli Sci related internships, I suspect DC beats Boston easily in terms of opportunities. The same is probably true for Econ.
For what it’s worth, at Tufts I think your child would be in the midst of an overall harder-working, more high-achieving group of students. This said, my son has found Tufts very collaborative, friendly, and outgoing. I should also mention that he has a very diverse set of friends at Tufts–in all dimensions. Yes, there are lots of kids from well-to-do families but there are also lots of kids who aren’t.
Good luck with the decision-making!