OAF Page Customization

First the basic difference between personalization and customization !
Personalization is oracle provided flexibility to change standard behavior as personal need. using personalization we do not require to write any custom code.while Customization is process to write custom code to fulfill our need.

Below are the 3 basic components of OAF pages.
1) View Object (VO) ,
2) Controler (CO) and
3) Application Module (AM)  , components in  MVC architecture.    

in OAF Customization we need to write custom code for any of the above 3 based on the requirement.

Now checkout the existing CO or AM name (From About this page at the left below of each html page) and download the class file then ftp to your local system.

(assume QuickUpdateCO.java is standard file)

use any decompiler and get the source java code and follow below steps -
1) Rename the standard CO/AM (QuickUpdateCO.java) with custom name XXXXXQuickUpdateCO.java
2) extend the standard CO/AM
3) import the standard package
4) define constructor for the custom program
5) define the package name (location where code resides)

package xxxxx.oracle.apps.pos.supplier.webui;
import oracle.apps.pos,suppliers.webui.QuickUpdateCO.java;
XXXXXQuickUpdateCO.java extends QuickUpdateCO.java;
{
XXXXXQuickUpdateCO{};
ProcessRequest(){
super();
};
ProcessFormRequest{
super();
};
} 

6) Write code in processRequest (if customization need to be invoked on page display) or in processFormRequest (if customization required on any event on the page).
7)FTP the new java code and compile using javac XXXXXQuickUpdateCO.java
8)Place the class file at xxxxx.oracle.apps.pos.supplier.webui (package defined at top of CO)

CO -:
a) Click on personalization link
b) Click on pencil button against page layout property
c) Give the new controller name along with package name at the desired level 
d) Tab out and apply

VO/AM-:
Oracle does not provide option to extend the VO and AM using personalization.Though there is concept of SUBSTITUTION which is being used to extend the standard VO and AM.
You can use JDeveloper to generate the substitution and then import substitution using  jpximport .