Funnily I had the same question at the Scottish 6-days. Basically, you can check the website of the French federation, which has a calendar (yearly published, so not necessarily up to date, but still quite complete) as well as contact for all clubs. For Toulon, I would recommend to contact Le Pradet club (POLES), which is a good club in southern France.
http://ffco.asso.fr/site%20guigui/pagecalendrier.h...
http://ffco.asso.fr/site%20guigui/pagecarte.htm
Then you have
http://www.matrace.fr/calendrier.php which has a calendar dedicated to O events and other nature sports, as well as being a routegadget server.
Biggest events are on woo calendar. If you are in France around the end of October, there will be 5 days of orienteering in Provence with the sprint and night championships. (for once, the english part of the website seems not so bad)
http://provence2009.com/?lng=en