If you have predefined values for your fields, you can use "default" attribute on your fields or "default_get" method to set the default values to the fields.

If your fields depends on other field(s), you can either use "compute" or "onchange" methods to set the value.

-------------------------------UPDATE-------------------------------------

    @api.multi
    def action_add(self):
        for rec in self:
            rec.result = rec.number1 + rec.number2

    @api.multi
    def action_multi(self):
        for rec in self:
            rec.result = rec.number1 * rec.number2

Call these methods from your buttons. This way you can use the fields for the calculation and store the value in another field. Make sure your fields must be of type either integer or float if you are doing mathematical calculation.