How to identify a Catholic 'order' from initials that follow a nun or priest's name? I am regularly cataloguing publications written by (or about) sisters and priests. I am unfamiliar with all of the different Catholic orders. Is there an online list or reference that identifies what the initials after their names stand for?

For example:
S.J. = Society of Jesus
O.S.U = Ursuline Sisters of the Roman Union
Here is a partial list of orders with members who are/were bishops. That is this list contains only males religious societies.

Here is another 800 abbreviations including women religious.

Annuario Pontificio lists all the existing approved and recognized Catholic religious orders. You can see the 2011 list here.

That should help you to some extent. But please note that it is nearly impossible to list all religious organizations that exists now and has existed before. Religious organizations in their early stages can be limited to only certain diocese to function under the jurisdiction of a diocesan bishop.