SQL> select open_mode from v$database;

OPEN_MODE

--------------------

MOUNTED

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

Database altered.

SQL> ALTER DATABASE OPEN;

Database altered.

SQL>  select open_mode from v$database;

OPEN_MODE

--------------------

READ ONLY

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT;

Database altered.

SQL> select open_mode from v$database;

OPEN_MODE

--------------------

READ ONLY WITH APPLY