1. Before you start, make sure that the appropriate checkboxes are enabled in Server GUI, Admin Tool, Enable logging ( possibly include db session stacktrace )
This is needed to add additional logging to the database sessions we are interested in.

2.

When the application server is hanging, open a SQL session using a SYSDBA account and execute the following query. 
It will show the open connections and some logging info we can use. 
Mainly how long sessions are waiting, the sql statement and the originator in the server code.

SELECT V.SID, V.MODULE, V.CLIENT_IDENTIFIER, V.ACTION, V.CLIENT_INFO, SQL.SQL_TEXT, V.STATE, V.SECONDS_IN_WAIT FROM V$SESSION V, V$SQLAREA SQL WHERE V.SQL_ADDRESS=SQL.ADDRESS AND V.MODULE LIKE 'ABS%' ORDER BY V.SECONDS_IN_WAIT

If you cannot use a sys adm account, you can use the following statement, but it will not list the actual sql statement, only the ids. 

SELECT V.SID, V.MODULE, V.CLIENT_IDENTIFIER, V.ACTION, V.CLIENT_INFO, V.STATE, V.SECONDS_IN_WAIT FROM V$SESSION V WHERE V.MODULE LIKE 'ABS%' ORDER BY V.SECONDS_IN_WAIT

You do not necessary need to wait til the app server is hanging, It can be executed before as well to see if sessions are waiting. 

3. To check for blocking sessions, here are a couple of useful queries

	1.	See if sessions are blocking other sessions
		select l1.sid, ' IS BLOCKING ', l2.sid
			from v$lock l1, v$lock l2
			where l1.block =1 and l2.request > 0
			and l1.id1=l2.id1
			and l1.id2=l2.id2