Dear Navaneethan,
Greetings of the day
Yes, you can use external login instead of using default login of OutSystems.
You can remove call to default login method from login button from screen and then call your required rest API.
After getting response, you can create functionality accordingly.
Just to mention, you will not be able to use user related functionality after using your own login mechanisms.
Check following URLs from OutSystems related to External Login.
Questions regarding external login | OutSystems
Use an External Authentication Provider - OutSystems
Let us know if these help you achieve what you are looking at.
Regards,
Palak Patel