_order = 'field_name desc' in the model should normally work but it applies for all views of your application
If you want to specify the order only in that many2one field, one solution is to use a context value and overwrite the search function:

f.i.:

In my python class:
    period_id = fields.Many2one(string='Period',
                                                       comodel_name='account.period')

In my XML view
    <field name="period_id"  context="{'order_display': 'name desc' }"/>

In the python model of the comodel:
    class AccountPeriod(models.Model):
        _inherit = 'account.period'
        @api.model
        def search(self, args, offset=0, limit=None, order=None, count=False):
            ctx = self._context
            if 'order_display' in ctx:
                order = ctx['order_display']
            res = super(AccountPeriod, self).search(
                args, offset=offset, limit=limit, order=order, count=count)
            return res