OpenID Connect, redirect without login form if not already logged in?
You are considering SSO behaviour on-top of IDP. This is usually outside OpenID Connect specification and usually bound to specific identity provider you are using (ex:- Azure, PING or WSO2). But there are some parameters to tweak the this behaviour such as prompt and login_hint which are optional.
From OpenID Connect authentication request section
prompt
Space delimited, case sensitive list of ASCII string values that specifies whether the Authorization Server prompts the End-User for reauthentication and consent.
Valid values are login, none, consent and select_account. You can use them to enforce force login or to allow a select account.
login_hint
Hint to the Authorization Server about the login identifier the End-User might use to log in (if necessary)
One good example is enabling SSO behaviour by passing login_hint to identity provider. If identity provider can verify identity against (for example) a corporate LDAP and detect logged in state, you can give credential free login experience. At the same time, you may use prompt=login to enforce a login, even when identity provider hold a logged in session.