It's quite simple :)
<apex:inputField value="{!Order__c.Account__c}"/>
But maybe better will be to modify it a bit. Change the definition of order
variable in apex to this:
public Order__c order {get;set;}
I'm not sure why you marked it as private (= can't be changed from Visualforce) and final (used rather in places where you don't want to change something at all).
And then in Visualforce you can refer to it like that:
<apex:inputField value="{!order.Account__c}"/>