Tracing can be enabled at the Session level.

alter session set sql_trace=true;

After configuring the Session, a Trace File is generated in the User_Dump_Dest path, where the Trace File is named with the (process ID number of the server process).

Now, a Trace File is created when a SQL Statement is executed, and the file is stored in the User_Dump_Dest path.

The database administrator can also enable Tracing for a specific User Session through the procedure Session_Trace_Enable available in the DBMS_MONITOR package, after identifying the Session using the (Session Identifier & Serial Number).

Let's assume now we want to put the Session for the user OBAY in Tracing mode.

Select Sid, serial# from v$session where username='OBAY'; 

Now, using this Session information, we can enable it in Tracing mode.

begin
dbms_monitor.session_trace_enable( session_id=>841,
serial_num=>27);
end;
/ 

You can also disable this mode through the procedure Session_Trace_Disable.