Yes it certainly helps cleaning the session data.
Space occupied by any specific session mainly depends on the Virtual user logging for the session, Extended logs always take considerable amount of space.
If you have logging enabled for every session, probably clearing only the VULogs for the sessions might help.
COUNTERRUN and VULOG might the tables of interest for back-up and cleanup exercise.
1. I deleted 25% old records(10 million out of 40 million records) of OLT.COUNTERRUN, which is 1.2G .
But still available space it says is 40 MB.
2. Also I cannot find VULOG table, can you suggest the schema name. OATS I am using is 9.x
You must be checking the Administration Storage page. Just check the space occupied by various tablespaces (Home>Administration>Storage>Tablespaces).
You can see the Percent Used , Allocated (MB) and Used (MB) for each of the tablespace SYSAUX, SYSTEM, UNDO and USERS, which gives you the clear picture.
VULOG table should also be under the OLT schema.
If you have enough space available under the USERS Tablespace, I don't see any issue performing further run, apart from this all the tablespace are Auto Extensible.