You do know that you will need to replace a larger section than a 1' square? Roof trusses are typically on either 16" or 2' centers.
An alternative to replacing the roof sheet might be to use an epoxy like Git Rot to reinforce the problem area. If you can get the OSB thoroughly dry, the epoxy products work well.
When you reassemble your repair, using Eternabond tape on the seam will insure against future water intrusion. I discovered the Eternabond tapes through this Forum and now have all my roof penetrations, seams, and even aluminum trim strips covered with the tapes. I test every once in awhile, using the pressurized interior test, to see if I have developed a leak. So far, the tapes have done a great job.