Did a quick google and found this: http://www.mcconveycycles.com/store/product/18622/Giant-Fsa-Overdrive-2-Od2-Mtb-Headset/
Doesnt state the top bearing sizes but the part number leads to here:
http://www.thindustries.com.tw/?language=zh_TW&mod=7&class=1&mod_class=0&action=read&item=3
MR136 has an OD of 41.8mm and an OA of 45deg.
If the normal on one mixer headset has the same outer bearing dimensions then you should be able to use the bearing and split ring from the giant headset but obviously the top cap may be a problem as it may not fit the on one headset. Superstar and Hope also do headsets that will work in the On One frame.
If the Giant headset cup is the same OD (44mm) then it would fit.
edit: just looked up a Trance X on cane Creeks wesbite and it says the top is a 44 and the bottom a 56 which suggests the same zero stack top cup but a different (larger frame bore) zero stack 1.5″ bottom cup.
So you should be able to us an external 1.5″ lower cup ie an on one mixer, superstar tapered or Hope external along with a Giant OD2 top cup.