Other apps are not Out systems app.they are non OutSystems web application.So is it possible to do Seam less login from OutSystems to a non OutSystems web page?

I think you need a Single Sign on Solution to all the apps if you want users to seamlessly login to all the apps. E.g. If you have ADFS or Azure AD login setup, you could use that to seamlessly login into OutSystems apps and non OutSystems apps. OutSystems allows you configure these directly onto OutSystems User Manager. Please read here for more information.

If you dont have any existing single sign on solution, You can look at different options like OKTA, AUTH0, Azure AD rather than saving username and password which is very bad solution in terms of security.