As I surmised, the Minimum Order plugin (version 7) doesn't 'know' OPC. You'll need to edit /includes/classes/observers/class.minimum_order_amount.php in two spots.
First, on line 24, add the highlighted fragment:
Code:
$zco_notifier->attach($this, array('NOTIFY_HEADER_END_SHOPPING_CART', 'NOTIFY_HEADER_START_CHECKOUT_SHIPPING', 'NOTIFY_HEADER_START_CHECKOUT_PAYMENT', 'NOTIFY_HEADER_START_CHECKOUT_CONFIRMATION', 'NOTIFY_HEADER_START_CHECKOUT_ONE'));
Second, add that notification to the list in the section that starts on line 67:
Code:
case 'NOTIFY_HEADER_START_CHECKOUT_SHIPPING':
case 'NOTIFY_HEADER_START_CHECKOUT_PAYMENT':
case 'NOTIFY_HEADER_START_CHECKOUT_CONFIRMATION':
case 'NOTIFY_HEADER_START_CHECKOUT_ONE':