You can't currently automate the actual Synchronize tool in Opus but you can automate the similar (but less powerful) "Copy UPDATEALL" command, with something like this:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd Copy FILE "C:\Folder I Want Backed Up" TO "G:\Backups Place" UPDATEALL
It's not ideal, though. If something is deleted from the source it won't be deleted from the destination. More importantly, you'll get progress dialogs, and possibly error messages, appearing on your screen whenever the sync triggers. I'm not sure how well it will work if you schedule the sync to happen when not logged in.
For automatic, scheduled and unattended copying/synching I would say you would do best to use something designed for that. The Offline Files feature of Windows sounds like what you are looking for (at least the version in Vista; I haven't looked at how it works in previous versions of the OS).
There are lots of file-based backup programs which are designed around what you want to do, too. (Some better than others. I tried a lot of them a few weeks ago and wasn't happy with any of them, but I have particular needs when it comes to file permissions, backup accounts, and unicode support.)