Regardless of ITU, America, or what/who ever, all optical networks of this scale, no matter who makes the gear used on them, have OSC between nodes. If they dont, the nodes get isolated and can no longer be managed unless they have an entirely separate out of band management connection.
As already stated, the OSC is entirely separate from all other traffic carrying channels, running on its own dedicated wavelength and bypassing any active equipment such as amplifiers etc. They also tend to run at a much slower speed, 100-155mbit because of a number of factors not limited to: greater unamplified reach, they dont need to carry that much traffic, and the processors that manage the nodes arent very powerful anyway.
Some of these networks cross areas of country where there may be no other means of connectivity around, e.g. across the Nullarbor in Australia. You arent just going to pick up a DSL or other kind of connection to provide connectivity to your optical site - heck, a lot of them are even solar powered because you cant get a connection to the electrical grid. Therefore, OSC runs "in-band" on the same optical fibre used to carry the rest of the traffic, because thats the only way you'll get a management network out to these very remote sites.