“1) Do people actually affiliate themselves with OO?”
Yes, but the number of people is very low. If you look at the alumni page of YCT you’ll see they have placed people at a few shuls (usually small out of town shuls), though most of them end up with other jobs like at Hillels. Presumably at least some of the people at their shuls would consider themselves OO.
“2) Is it an obvious thing (like perhaps as someone would be able to tell who is Chassidish, Yeshivish, or MO based on appearance, shuls, or hashkafah)?”
Appearance: No. Shuls: only indication would be YCT rabbi (however, not all YCT rabbis are necessarily OO — see below). Hashkafa: see below.
“3) What’s the difference between MO and OO?”
MO means accepting traditional haskhafa and halacha 100% while being more open to university education, popular culture, involvement in modern society, perhaps even exploring the varying views in controversial areas such as Chazal and science, etc.
OO means affiliating with an Orthodox shul, having a mechitza, identifying as Orthodox, and yet being more flexible about certain things (such as Biblical criticism, women’s participation in shul, attitude toward non-Orthodox, etc.) Some individual OO rabbis have advocated or done very Reform-ish things, like saying you don’t have to say shelo asani isha.
Some OO are more or less the same as left-wing MO while some are like Conservative or Reform in their hashkafa (one, for example, claimed Moshiach will not necessarily be an actual person, chas v’shalom).
I actually know one YCT-ordained rabbi who is hashkafically and halachically totally MO, not OO, and who is accepted as an Orthodox rabbi by all the other Orthodox rabbis in town. Even within YCT there is a lot of variety.