Case 1: A multiplexed copy of the missing log is available.

If a redo log is missing, it should be restored from a multiplexed copy, if possible. 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 131555128 bytes
Fixed Size 454456 bytes
Variable Size 88080384 bytes
Database Buffers 41943040 bytes
Redo Buffers 1077248 bytes
Database mounted.

ORA-00313: open failed for members of log group 3 of thread 1
ORA-00312: online log 3 thread 1: 'D:\ORACLE_DATA\LOGS\ORCL\REDO03.LOG'

SQL> To fix this we simply copy REDO03.LOG from its multiplexed location on E: to the above location on D:.

SQL> alter database open;
Database altered.

SQL>
That's it - the database is open for use.