Until recently, the OS .Net Platform required sessions to be stored in an SQL Server database: This meant that, even if your Platform model was hosted in an Oracle Database, you still had to have a machine running SQL Server to store the sessions (the SQL Server Express edition was sufficient for this, however).
From OutSystems Platform version 4.1.3.26 onwards, however, you can now configure Oracle-based sessions. To do so, simply follow these instructions:

1) Run all the steps in the installation checklist, until the step where you are to create the Service Center database.
2) After this, locate the session_oracle_creation.sql script at <Installation Folder>\db, and open it with a text editor (e.g. notepad);
3) By default, this script configures the following settings:
Tablespace: OSSTATE;
User: HUBSTATE
4) If needed, change the names of the tablespace OSSTATE and/or the user HUBSTATE to a more appropriate one; otherwise, just leave things as they are; if you change any of these settings, remember to do so throughout the file);
5) Customize the datafile location of the OSSTATE tablespace (default C:\ORACLE\ORADATA\OUTSYSTEMS_STATEDATA.ora);
6) Customize the HUBSTATE password (replacing the string &hubstatepassword by something else, e.g. ‘outsystems’ ; don’t forget to include the single quote marks);
7) Run this script like you did with the script to create the data model of the OutSystems Platform;
8) Proceed with Setup until the Session State Configuration in the Configuration Tool;
9) To configure Session State, in the session tab, use the following information:
DBMS: Oracle;
SID: <the one for the database where you ran the script in step 7.>
User: HUBSTATE (or the new name, if customized);
Password: outsystems (or other, if customized);
Tablespace: OSSTATE (or other, if customized);
10) Press the Test Login button to confirm that the settings are correct;
11) Press the Configure Session Database to insert the session model information into the database.
12) Proceed with the installation following the checklist.

Hope this helps.

Regards,

Miguel