OSM data doesn't contain an easy way to find the top 1000 cities. You'd end up with 100.000s. Looking for wikipedia tags, population (which often comes from Wikipedia) and 'admin' tags might be a good guide.

(I work on a OSM geocoder, not offline but has a Python library http://geocoder.opencagedata.com/)

That is interesting. I haven't looked at that data set. At the moment, this only looks at cities with a population > 1000 obtained from GeoNames.