DGS has a a couple of sections in their FAQ dealing with this question. The short version is:
You CAN use all kind of aids by doing research of your own.
For example studying books, looking up joseki dictionaries, game databases, and analyzing your game in an SGF editor.
In light of fair play you SHOULD NOT ask another person or go-playing program to help you find your moves or strategy in the game.
That’s a pretty typical example of correspondence play guidelines. Keep in mind, though, that on OGS some tournaments have analysis disabled. While there is no consensus, in such tournaments you’re probably better not to use any aids.
Personally, I do sometimes consult josekipedia or fuseki database, especially when I’m against stronger players. And while I do not use engines myself, as long as my opponent has a more or less accurate rating, I do not particularly care whether he/she/it is a human, computer, or combination of both with some books on top of it. 