All it says in the rules is that any laptop device connected to the OI must be on battery power, does this also apply to the OI itself. Do I or can I make a battery bank of 12V batteries to the OI or is there literally just a dc plug provided at the match?
The OI gets power through the competition port at competition.
You can power the OI using batteries, but it isn’t needed for competition.
I use a 9.6v RC car rechargeable, and an adapter to connect it to the OI. You can get everything you need at RadioShack for <$20.