Enabling and Disabling of maintenance mode can be done through ADADMIN.
But the same can be also done from executing SQL scripts as below.


Enabling Maintenance Mode:

sqlplus apps/appspassword

SQL>$AD_TOP/patch/115/sql/adsetmmd.sql ENABLE

Disabling Maintenance Mode:

sqlplus apps/appspassword 
 
SQL>$AD_TOP/patch/115/sql/adsetmmd.sql DISABLE