When you say OMF mixdown, do you mean transfer? OMF is a file interchange format that will allow your tracks to be transferred to and from video and audio workstations. However, keep in mind that it's not a perfect science and that going from Nuendo to Pro tools can be painful if you don't use the right tools. Please check out this website.
http://www.nuendo-post.com. You will find the process is described in an article by Steve Tusher. It's worth the read because it will give you a method that has worked for people. Plus you can get feedback from others in the forum if you still have problems.

Best of luck!

Steve