In the default 'atomlist.dat' file, there is an entry, [.OW. O]. Presumably, yours should be added as: [..OW O].
DG3 and DC5 have already been included in 'baselist.dat' (as 'g' and 'c' respectively). What specific message does 'find_pair' output for your structure? Did you know that, as of 3DNA v2.1, you do not have to modify 'baselist.dat'? Nevertheless, it is a good idea to be explicit about each mapping.
Attaching a sample PDB file would clarify the issues.
Xiang-Jun