Here is the simple query, showing who is connected to the server.

col username FOR a15 col program FOR a25 trun SELECT sid, serial#, username, program, SUBSTR(module,1,10) module, logon_time FROM v$session ORDER BY 1,2;

This is almost the same query, limiting the output to the active users:

SELECT sid, serial#, username, program, SUBSTR(module,1,10) module, logon_time […]