Officially, the OEM version needs to be tied to a single PC and cannot be transfered to another. Usually that is the motherboard, since that contains most of the devices. Adding a SSD (and thus moving/reinstalling Windows to it) is same as changing HDD, or simply reinstalling.
So you can reinstall windows to the new SSD (easiest solution) or move the current installation to the ssd (harder). The latter needs cloning software and some minor tweaks good for SSDs (these are done in the former case if Windows detects an SSD installation target).
Usually an OEM key will activate online ok, unless the new installation is very different (I really don't know how much difference counts) and you already activated in the past 6 months (at least that was an unofficial rumor for XP). In either case, you can activate by call.