I have a other solution, not tested but in theory work fine.

In your object columns add

'id': fields.integer('ID',  readonly=True),

That will force openerp to add ID field in the object column property

After in your view, add

<field name="id" invisible="1"/>

That will but the ID field invisible on your form, and allow the next change to work

<field name="m2o_field" attrs="{'readonly': [('id','!=',False)]}"/>

That will put the field on readonly when the record will be create. So dont forget to put required=true for the m2o_field, because if you forget to select a value, after creation you cant change it.

I prefer this solution, because no need to create a new field, no need to manage field to set it when record create.

Give me a feedback if you do it, please

You can block the value change of any many2one fields with widget attribute like, widget="many2one_list" example : <field name="id" widget="many2one_list"/> its gives a list formate and you cant change it

like this you can set other fields :one2many_list : many2one_list