.ODC is the extension for the dialog control file used by ObjectDCL and OpenDCL.
The file opens into an editor similar in appearance to the VBA IDE.
If you have the old ObjectDCL editor [2004] it should work.
If you have the OpenDCL editor [currently beta 9] it should work.
I'm not sure when the new ObjectDCL editor will be available.
Just be sure so save a copy before you start to play.
