Hi Robson

You make the second field to compute (wiil be readonly automatically , you may know)

def _foo(self):
     # On2many field will have a related Many2one field to parent object, example parent_id
     bar = # Calculate the parent_id
     for i in self.env['model of first field'].search([('parent_id', '=', bar )]):
         # You can get all lines of One2many field of that parent object
         # Find the value
     self.second_field = value to be passed

second_field = fields.Float(compute='_foo')

All the best