I am developing a web server application and would like to implement the Authorisation Code Flow using openid-client.
Can I use the OAuth 2.0 Playground (https://www.oauth.com/playground/) to simulate the Authorisation Code Flow in my application and test the redirection and stuff like that?
This is a simulation to walk you through the process to help you learn the steps.
I would suggest a better option would be to utilize one of the provider's free trials or free tiers.
The site you linked is sponsored by Okta/Auth0, and they have a free option. I would stick to the Auth0 side unless you have a specific reason to use the Okta side, much more dev-friendly.
And, there are others that are dev-focused (although, I haven't validated how easy or featured they are) like WorkOS, Frontegg, Clerk, and plenty more.
I'd probably also avoid Microsoft/Azure/Entra and AWS IAM/Cognito as they're a little obtuse or specific in their feature-set. Keycloak is a nice open source project, but it can be a bit finnicky.
You could even do a sign-in with Google since that uses OIDC with the authorization code flow. But, you don't get control of the IdP side if you need to customize and test further things out.