Below commands can be used to open a database in reset logs mode.
SQL> connect / as sysdba
SQL> shutdown immediate
SQL> startup mount
SQL> recover database until cancel using backup controlfile;
SQL> cancel
SQL> alter database open resetlogs;
SQL> connect / as sysdba
SQL> shutdown immediate
SQL> startup mount
SQL> recover database until cancel using backup controlfile;
SQL> cancel
SQL> alter database open resetlogs;