I can't say for sure what it means, but your guess looks like it's correct.
Regardless of what it means, the "OT" is an annotation that was added some time after the census was recorded, maybe by a worker in the census office or maybe by a later researcher. It looks like they marked all of the female heads of household that way. They also marked "OT" for adult women who were identified as servants. The adult women who are identified as "Wife" or who appear to be married and living with a husband are annotated "Wf." It looks like the markings were part of an analysis to determine what fractions of women were living with husbands and without husbands. Interesting, but not a census detail that we need to pay attention to for genealogical purposes!