Which country do you fill the TWOV in?

I have a flight ORD (Chicago) - LAX(Los Angeles) - PEK (Beijing) -OKA (Okinawa) then a return flight from OKA-PEK-ORD. After researching the TWOV I'm still confused about in which country I would fill out the TWOV. While I'm at the LAX airport going to PEK? Or when I arrive in PEK and get off the plane? Could you please help?