Hi,

To add a one2many field in odoo, you can refer this video explaining the same in details, see: How to Add One2many Fields In Odoo

Example:

class HRDepartment(models.Model):
    _inherit = 'hr.department'
    
    employee_ids = fields.One2many('hr.employee', 'department_id', string='Employees')


class HREmployee(models.Model):
    _inherit = 'hr.employee'

    department_id = fields.Many2one('hr.department', string='Department')

Thanks