Actually, iMovie is separate from OS X, though iMovie and the other iLife applications do come standard with most Mac machines by default. If not, it can be purchased Nd downloaded on the Mac App Store if you're using a supported version of OS X, or old versions can be purchased on disc on Amazon and other sites.
As for using it to edit DVDs, I'm sure iMovie could indeed be used, though it may force you to copy the video content to a single file and create a new DVD from scratch. For the copying of a DVD to a single video file, a program like Handbrake will work.
Exactly how much do you need to edit the DVD?