OCAU has a tradition of posting April Fools jokes on the news page shortly after midnight on April 1st. These usually take the form of dramatic policy change announcements, which often provoke an irate response from people who don't realise it's a joke.
Here's the pranks from the past:
2007: OCAU's front page was blank except for the message "sauce code hacked by mmx bandits". Visitors were then immediately redirected to a random page about cats or kittens.
2006: Back to the Good Old Days! - many forums to be deleted, no more new members, forum accounts under 2 years old to be deleted.
2005: Agg was away at World Superbikes, so no official prank, but Mred32 had a weird problem with the news page which confused a few people anyway. :)
2004: Become an OCAU Subscriber! - OCAU announces plans to charge outrageously for access.
2003: Moving on.. - Agg sells OCAU to TweakTown, so he can pursue his dream of writing a romance novel.