According to
this website (also in Japanese) OSS started today (13 May) but only for passengers arriving on 17 flights operated by 8 airlines (JL, NH, DL, UA, AA, HA, SQ, KE). It will be gradually extended to cover all 50 or so flights arriving from the US.
Originally Posted by
hamburgler
... though it will be interesting to see how many flights are left after 24 frequencies go to HND next year (12 US carrier, 12 JP carrier)
According to the figure mentioned above about one half of the current US arrivals will still remain in NRT.