O2 dongle will most likely run on normal sim - so use any link from the members in this forum to order a giffgaff sim.
just remember to buy a GIGAbag for your dongle, as GOODYbags are not allowed for dongles.
wont let me says ive already got an active sim
order a sim from your own mygiffgaff page : https://giffgaff.com/orders/mgm
that way you'll also get the £5 bonus once it gets activated ;-)