You can also turn off new account creation in the OAuth settings - see https://docs.moodle.org/36/en/OAuth_2_authentication#Preventing_new_account_creation - and then people will be prompted to link it to their existing account (the ldap one).
And you can link login accounts - see https://docs.moodle.org/36/en/Linked_logins
And you can link login accounts - see https://docs.moodle.org/36/en/Linked_logins