Each purchase of OOTP may be installed on up to TWO computers using the same operating system class. That is, you can install two copies on PCs, two copies on Macs, or two copies on Linux. (For example, Windows XP and Vista both count as "PC licenses.")

For OOTP 10 only, Mac and Linux are considered the same operating system class. That is, you can install one copy on a Linux box, and another copy on a Mac.

You cannot install OOTP on one PC and one Mac, unless you are running an emulator of some kind (like Boot Camp) that allows you run native PC programs on the Mac. In this case, you are effectively installing the PC software on both machines. Note: This is not an officially supported configuration. Use at your own risk!