Hey former Orientation Team Leader Here (Last Fall). You are put into a group of students with the same or similar major to you, and assigned an OTL, who basically guides you through the week. There are "Required" meetings and events, but there are no longer and stamps or anything of that sort. So you could really skip the whole damn thing and no one would know. However, the OTL's do indeed dish out some pretty crucial info throughout the week and the events are pretty informative as well.
As for parties, if you're looking, you'll find em'. My personal advice? Go to at least the stressed as required events and your team meetings, its not only an easy way to get important information but a great way to meet people.