public with sharing class ManageReturnCX{

    private ApexPages.StandardController stdCtrl;
    public Order_Form__c ordform {get;set;}
    public Order__c ordlist {get;set;}

    public ManageReturnCX(ApexPages.StandardController std){
        stdCtrl = std;
    }

    public void OrderPopulated(){
        Return__c ret = (Return__c) stdCtrl.getRecord();


        ordform = [SELECT Id, Name, Purchased_Date__c FROM Order_Form__c WHERE Id=:ret.Order_Form__c];
    }

}

<apex:page standardController="Return__c"  extensions="ManageReturnCX" sidebar="false" showHeader="false">
   <apex:sectionHeader title="Return Entry"/>
   <apex:form >
      <apex:pageBlock title="Return Entry">
         <apex:actionRegion >
            <apex:pageBlockSection title="Return Header" columns="2">
               <apex:pageBlockSectionItem >
                  <apex:outputLabel value="Order Name"/>
                  <apex:outputPanel >
                     <apex:inputField value="{!Return__c.Order_Form__c}">
                        <apex:actionSupport event="onchange"
                           action="{!OrderPopulated}"
                           rerender="thePageBlock"
                           status="status"/>
                     </apex:inputField>
                     <!--<apex:inputField value="{!Order_Form__c.Purchased_Date__c}"/>-->
                     <apex:actionStatus startText="applying products..."
                        id="status"/>
                  </apex:outputPanel>
               </apex:pageBlockSectionItem>
            </apex:pageBlockSection>
         </apex:actionRegion>
         <apex:pageBlockSection title="Items to be Returned" columns="2" id="thePageBlock" rendered="true">
            <apex:pageBlockSectionItem >
               <apex:outputLabel value="Product Name"/>
               <apex:inputField value="{!ordlist.Product__c}"/>
            </apex:pageBlockSectionItem>
         </apex:pageBlockSection>
      </apex:pageBlock>
   </apex:form>
</apex:page>