I'll note that postgres is (sort-of) a storage option for one of the (rather) early object databases, the much maligned, yet perhaps under-used zodb:

http://www.zodb.org/

Still evolving in its old age, along with postgres:

http://www.newtdb.org/en/latest/how-it-works.html

Other than that I'm aware of gemstone/s:

https://gemtalksystems.com/products/gs64/

Which can be paired with the maglev ruby implementation for similar (but presumably higher performing, more featurefull) experience with distributed objects in ruby:

http://maglev.github.io/

I suppose that with jsonb, and a node/js server, one could indeed argue that postgres is (again) an object oriented db - but I don't think anyone would push that argument too strongly.