This script shows active transactions… and how many blocks are needed to rollback (it’s good also, when you do a Shutdown Immediate… and don’t know how much time you need to rollback)
(in 9i Rollback appears in v$session_longops)

alter session set sort_area_size=5000000;
set linesize 232
col username format a12;
col sql_text format a40;
col used_ublk format 9999;
col used_urec format 9999;
col log_io format 99999;
col phy_io format 99999;

select
distinct (a.sid), a.username, substr(c.sql_text,1,38) as SQL,
b.start_time as START_DT, b.used_ublk as Blk_RBS, b.used_urec as Recs_RBS,
b.log_io as IO_ReLog, b.phy_io as IO_Phy
from
v$session a, v$transaction b, v$sql c
where
a.saddr=b.ses_addr and a.SQL_HASH_VALUE=c.HASH_VALUE(+);