I'm starting over again on my workstation so I just installed Bryce 5.5, I'll install 6.1 later. But in the mean time, There was a thread, which I cant seem to find, with some good tips on how to manage OBP's in Bryce without breaking Bryce.
 
I have some Models in OBP format, but not sure how to cleanly bring them into Bryce.

Have you tried the back room here is a link to the page.

http://backroom.renderosity.com/~bryce/brbrycelinks.htm

In Bryce, click on the arrow next to Create on the menu. Then go to the category you want to put the item in. On the bottom, you should see something like add, delete, import,  export. Click on import, and in the file menu, go to the folder or location you have the obp file. Select it,  and add it to your repertory.

Hope this helps...