To set up OTP authentication in Odoo 15 using the msg91 OTP provider, you will need to install the "msg91_gateway" module from the Odoo app store. Odoo MSG91 SMS Gateway Module provides required SMS gateway facility to "SMS Notification" application for sending SMS notification to any mobile number. The module is able to send text SMS directly to the customers, suppliers, buyers, partners or to any mobile number.

If you are developer then you can also develop the modules as per your need.