devtay
There is no special or essential reason to put contribs or add ons into /opt.
If you download the generic package from osCommerce site, you can unpack/install it wherever you want eg in /opt/shop or in ibay .../shop
For both initial setup & future maintenance, IMO you are better off to put a contrib like osCommerce into a ibay
eg named oscommerce or shop or onlineshop or as you prefer.
The main thing is to set ownership access rights on that ibay so that regular LAN users do not have access to it, & that is typically best achieved by ALWAYS creating a different group or groups who will own certain ibays, & then make the admin user or users for the osCommerce site a member of the group that owns the shop ibay (& disallow other users from having LAN access). This of course does not affect web access which is open to anyone.
Then download the generic latest version tar.gz file into the ibay & follow the generic Linux install instructions.
You want the latest version (that is compatible with the version of CentOS that sme is running (eg 8 or 9) so as to avoid the many security flaws etc that earlier versions of the contrib version has.
IIRC osCommerce has quite a good setup routine so follow the instructions carefully, & somewhere in the process make sure the MYSQL database is created.
You will also need to tweak ibay access php settings etc using some db commands, refer wiki.
Remember osCommerce is a BIG FULL FEATURED program, & there will be a big learning curve for you.
Think "system" when creating various components etc within osCommerce & think ahead when adding items etc re the best (or better for you) way to do things.
Similar concepts as above apply to any/most web programs/contribs/add ons for sme server.
I strongly advise you to subscribe to osCommerce security mailing list, & that way you are informed immediately of any security issues & bug fix releases & can update the osCommerce version you have installed on SME server regularly.
Keep your osCommerce version & SME server updated to avoid getting hacked.
Lastly, if you want to use add ons such as osCommerce, then I STRONGLY advise you to upgrade to SME 9.0 first, then run yum update to get latest packages etc, then install osCommerce.
That way you get the latest good quality server base system & will be able to install the latest stable version of osCommerce.
It is much better to have a very secure system when using php type web apps & SME 9.0 is the best starting point IMO.
There are many new features in SME9/CentOS6.5 that allow you to use different (multiple) versions of packages to best suit default packages vs contribs vs system stabiliity
eg
For Software Collections (versioning)
http://forums.contribs.org/index.php/topic,51223.msg259763.html#msg259763&
http://wiki.contribs.org/Software_collectionsFor Docker see
http://wiki.contribs.org/Docker