You are certainly looking for default values, domains or default values in the created object via a many2one. Your question could be a bit improved. So I gave you all what you might need.

Default values

In your python model you have your fields under _columns and you can also declare a dict name _defaults such as:

def _get_default_value(cr, uid, ids, context=None):
    return 'my field value'

_defaults = {
        'my_field1': _get_default_value,
        'my_field2': 'my field value',
         }

Have a look in the addons code for more exemples. And here is the official doc about fields:

Domain restriction

For domain, you can restrict the selection for the user using the strange [('field', '=', <value>)] notation.

Just add a domain on your field.

_columns = {
        'my_field': field.many2one(..., domain=[('field', '=', <value>)]),
        }

Setting default values in related object

And for setting values on creation of an object in a many2one, you can try to define values in the xml:

Here in a case of partners, for its contacts, the parent_id is by default set to the active id, this means the partner's contact.

<notebook colspan="4">
  <page string="Contacts" attrs="{'invisible': [('is_company', '=', False)]}">
    <field name="child_ids" context="{'default_parent_id': active_id}" mode="kanban">
[...]

In context, you must use default_XXX where XXX is the name of the field.

Objects, Fields and Methods
http://doc.openerp.com/trunk/developers/server/03_module_dev_02/