well it should be nls_date_format

setting nls_date_format will only take effect if you client side has NOT set NLS_LANG or NLS_DATE_FORMAT, if any of them is set then the client side predominates and will overwrite the nls_date_format you set in init.ora

you can check

VIEWs

nls_database_parameters
nls_instance_parameters
nls_session_parameters

nls_session_parameters predominates all above it then instance predominates database etc

I bet you have NLS_LANG=AMERICAN_AMERICA.WE8ISO8859P1 set in your client