VF Page:

<apex:page standardController="Order" extensions="O2I_OrderTest">
  <apex:form >
  <apex:sectionHeader title="New order" />
     <apex:pageBlock title=" Order Edit" mode="Edit">
     <apex:pageBlockButtons >
       <apex:commandButton action="{!save}" value="Save"/>
           <apex:commandButton action="{!cancel}" value="cancel"/>
         
            <!-- Calling controller actions by using action function when account and contacts are selected / changed  -->         
           <apex:actionFunction name="Selectcon" action="{!Selectcon}" reRender="PreparedForBlock"/>
           <apex:actionFunction name="Selectacc" action="{!Selectacc}" reRender="Addressblock"/>
         
     </apex:pageBlockButtons>
     <apex:pageBlockSection title="Information">
      <apex:inputField value="{!ord.QuoteId}"/>
       <apex:outputlabel >&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<b>Owner</b>&nbsp;&nbsp;&nbsp;{!$User.FirstName}&nbsp;&nbsp;{!$user.Lastname}</apex:outputlabel>                  
       <apex:inputField value="{!ord.Opportunityid}"/>
       <apex:inputField value="{!ord.O2I_So_Delivery_Date__c}"/>
        <apex:inputField value="{!ord.Accountid}" required="true" onchange="Selectcon()" />
         <apex:inputField value="{!ord.Status}"/>
         <apex:inputField value="{!ord.ContractId}" required="true"/>
       <apex:inputField value="{!ord.O2I_So_Order_to_Invoices_settings__c}" required="true"/>
       <apex:inputField value="{!ord.PoNumber}"/>
       <apex:inputField value="{!ord.EffectiveDate}"/>
       <apex:inputField value="{!ord.Description}"/>                 
    </apex:pageBlockSection>
  
     <apex:pageBlockSection title="Prepared For" id="PreparedForBlock">
       <apex:inputField value="{!ord.CustomerAuthorizedByid}" onchange="Selectcon()"/>
       <apex:inputField value="{!ord.O2I_So_phone__c}"/>
        <apex:inputField value="{!ord.O2I_So_email__c}"/>
         <apex:inputField value="{!ord.O2I_Fax__c}"/>
     </apex:pageBlockSection>
   
     <apex:pageBlockSection title="Terms" columns="1">
       <apex:inputField value="{!ord.O2I_Terms_and_Conditions__c}"/>
     </apex:pageBlockSection>
   
     <apex:pageBlockSection title="Address" id="Addressblock">
       <apex:inputField value="{!ord.O2I_So_Bill_To_Name__c}"/>
                  <apex:inputField value="{!ord.O2I_Ship_To_Name__c}"/>
                  <apex:inputField value="{!ord.BillingStreet }"/>
                  <apex:inputField value="{!ord.ShippingStreet}"/>
                  <apex:inputField value="{!ord.BillingCity}"/>
                   <apex:inputField value="{!ord.ShippingCity}"/>
                   <apex:inputField value="{!ord.BillingState}"/>
                   <apex:inputField value="{!ord.ShippingState}"/>
                   <apex:inputField value="{!ord.BillingpostalCode}"/>
                   <apex:inputField value="{!ord.ShippingpostalCode}"/>
                   <apex:inputField value="{!ord.BillingCountry}"/>
                   <apex:inputField value="{!ord.ShippingCountry}"/>
     </apex:pageBlockSection>
  
   </apex:pageBlock>
  </apex:form>
</apex:page>

apex cllass:
public with sharing class O2I_OrderTest {

Public Order ord{Get;set;}
Public list<Order> ordr{Get;set;}
O2I_Order_to_Invoices_settings__c o2i;
  public list<SelectOption> ChooseCurrency{get;set;}
    public String selectedCurrency{get;set;}
    Apexpages.StandardController scon;
     Account acc;
    Contact con;

    public O2I_OrderTest(ApexPages.StandardController controller) {
     this.ord=(Order)Controller.getRecord();
      scon=controller;
       string curr;
          if(Userinfo.isMultiCurrencyOrganization() || system.test.isRunningTest()){
        if(!system.test.isRunningTest())curr=String.valueof(ord.get('CurrencyIsoCode'));
        ChooseCurrency = new List<SelectOption>();     
        chooseCurrency.add(new SelectOption(Userinfo.getDefaultCurrency(),Userinfo.getDefaultCurrency()));
        Schema.sObjectType sobject_type = Account.getSObjectType(); //grab the sobject that was passed
        Schema.DescribeSObjectResult sobject_describe = sobject_type.getDescribe(); //describe the sobject
        Map<String, Schema.SObjectField> field_map = sobject_describe.fields.getMap();
        List<Schema.PicklistEntry> pick_list_values =  (!system.test.isRunningTest())? field_map.get('CurrencyISoCode').getDescribe().getPickListValues() : Account.Type.getDescribe().getPicklistValues(); //grab the list of picklist values for the passed field on the sobject
        for (Schema.PicklistEntry a : pick_list_values) { //for all values in the picklist list
        if(a.getValue()!=Userinfo.getDefaultCurrency())
        ChooseCurrency.add(new SelectOption(a.getValue(),a.getlabel()));//add the value to our final list
            }
          
          
        }
    }

         
            Public pagereference Selectacc(){
    if(Ord.AccountId!=null){
        acc=[Select Name,BillingStreet,BillingCity,BillingState,Billingpostalcode,billingCountry,Shippingstreet,Shippingcity,shippingstate,shippingpostalcode,shippingcountry from Account where id=:Ord.AccountId];
        ord.O2I_So_Bill_To_Name__c=acc.name;
        ord.BillingStreet=acc.BillingStreet;
        ord.BillingCity=acc.BillingCity;
        ord.BillingState=acc.BillingState;
        //ord..Billingpostalcode=acc.Billingpostalcode;
        ord.billingCountry=acc.billingCountry;
        ord.O2I_Ship_To_Name__c=acc.name;
        ord.Shippingstreet=acc.Shippingstreet;
        ord.Shippingcity=acc.Shippingcity;
        ord.shippingstate=acc.shippingstate;
        ord.shippingpostalcode=acc.shippingpostalcode;
        ord.shippingcountry=acc.shippingcountry;
     
        }
        return null;
    }
                  
         
           Public pagereference Selectcon(){
    if(ord.CustomerAuthorizedByid!=null){
        con=[Select Email,Phone,Fax from Contact where id=:ord.CustomerAuthorizedByid];
        ord.O2I_So_phone__c=con.phone;
        ord.O2I_So_email__c=con.email;
        ord.O2I_Fax__c=con.fax;
        }
        return null;
    }
   
}