Note with the new api you define a Selection field as follows:

from openerp import api, models, fields

class YourModel(models.Model):

_name = 'your.model'

my_selection_field = fields.Selection([('option1', 'Label 1'), ('option2', 'Label 2')], string='My Selection Field')

Using the new style gives the benefit of being able to extend the selection list by inheriting and redefining as follows:

my_selection_field = fields.Selection(selection_add=[('option3', 'Label 3')])