What you want is a sortkey; see http://www.mediawiki.org/wiki/Help:Cate ... a_category
For example, for Op.10 the sortkey might be "Symphonies, Op.10 (Various), 6" (moving the number to the end).
The reason you don't normally have to do this for work pages is because voodoo magic in the FTE template already moved the number to the end. You will, however, need to manually specify the sortkey if you manually include categories and you want it sorted differently.
P.S. I see the Oberthur pages have been renamed.