my first question would be....when you cant get your house over 70 degrees in the extreme cold, does your OWB ever get to the set temp and the blower shut off?
if the answer is yes then you could try turning up the off and on temps some. perhaps you need to have a larger heat exchanger in your furnace?
it is obvious that in the extreme cold you are unable to deliver enough btu to the home to compensate for the btu losses
first thing to check is...is your OWB capable of producing the needed btu and are your lines and exchanger capable of delivering those btu