If all or some redo log is missing. Here's an example, where I attempt to startup from SQLPlus when a redo log is missing:
SQL> startup
ORACLE instance started.
Total System Global Area 122755896 bytes
Fixed Size 453432 bytes
Variable Size 67108864 bytes
Database Buffers 54525952 bytes
Redo Buffers 667648 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: 'C:\O\ORADATA\REDO01.LOG'
SQL> recover database until cancel;
Media recovery complete.
SQL> alter database open resetlogs;
Database altered.
That's it - the database is open for use.
SQL> startup
ORACLE instance started.
Total System Global Area 122755896 bytes
Fixed Size 453432 bytes
Variable Size 67108864 bytes
Database Buffers 54525952 bytes
Redo Buffers 667648 bytes
Database mounted.
ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: 'C:\O\ORADATA\REDO01.LOG'
SQL> recover database until cancel;
Media recovery complete.
SQL> alter database open resetlogs;
Database altered.
That's it - the database is open for use.