I know about that.
They asked for datasets for all of Greece, and i’m sure there are no datasets that (even combined) can cover the whole Greece.

As far as i know, Geodata data are free to use and publish by everyone. The is no need of permission. http://geodata.gov.gr/content/about/

The same goes (except for commercial pusposes) for Oasa api. Thats why there is no api key.