Actually, xf86-input-synaptics is in Xenocara but the kernel peices are still missing from the tree.. so it is not yet ready.
Patches are occasionally put on tech@ for testing, hopefully they'll end up in 5.0.. people need to keep testing them or the progress may stall.
As stated though.. 4.8 is kind of old, you'll need to update to a snapshot if you want to attempt to get synaptics working, or wait and "hope" it gets enough testing by others.
Hint: That's not recommended.
http://marc.info/?l=openbsd-tech&m=130993646312118&w=2