When you click on Save button it calls two methods.

1. create: When you are creating a new record.

2. 'write': When you are updating a record.

   You can override these two methods by inherit the object.

   class stock_picking_out(osv.Model):
       _inherit = 'stock.picking.out'
   
       def create(cr, uid, vals, context=None):
           #your code
           res_id = super(stock_picking_out, self).create(cr, uid, vals, context=context)
           #your code
           return res_id
   
       def write(cr, uid, ids, vals, context=None):
           #your code
           super(stock_picking_out, self).wite(cr, uid, ids, vals, context=context)
           #your code
           return True
   

This is how you can override the methods.

If you want to see these methods in server here: def create and def write

Sudhir's answer is unfortunately wrong.

The "Save" does NOT call write if there was no change!
What is the correct answer?