Good point. No matter what precautions you take, if the power goes out you have a whole new set of problems. I'd ask someone, a neighbor or a friend, to check on the OWB a couple times while you're gone. A phone call from wherever you are as a reminder, too.
However, a full boiler and the set temp as low as possible should easily keep a system from freezing for a week or more, though. I can load mine right up full and get two full days out of it before there's just no more fire or coals to heat the water, but it's still at 135-140 degrees. Then you begin backfeeding heat from the water in your inside hot water tank until that temperature drops to the temperature of your cold water coming into the hot water storage tank from your main water supply (well or municipal). That could take another couple of days or more. If you keep the house heated with the oil burner, your cold water temperature will never drop below freezing, thus the OWB water temperature can never drop below freezing. Might be 45-50 degrees when you get back, but it will stay that way indefinitely as long as the house stays at that temperature.
Not that I have this problem - my OWB has a propane burner built right in it, which is my backup heat source when the fire goes out.