Track conference deadlines
While it does not send you reminders of upcoming deadlines or anything, I find that this site provides a very nice overview of the deadlines of a number of CS conferences:

http://confsearch.ethz.ch

You can get all conferences from a certain field, for example:
http://confsearch.ethz.ch/confsearch/faces/pages/topic.jsp?topic=Theory&amp;sortMode=1&amp;graphicView=1
gives an overview of theory conferences (I do not see Operations Research mentioned, and I am not sure whether it falls in any of the other categories they have).

It looks like you can also customize the overview to include a specific set of conferences (see their help page for more info).

According to the website, they obtain their info from DBLP and from user input (you can indeed modify information about a conference manually, thereby I am not completely sure how reliable the information on this site is).