I asked this question of 5/18:
Does anyone know what the abbreviation “OT” next to a name in the 1910 US census signifies? See, for example, the entry for Harriett HARRIS in the cropped attachment Harris.jpg. I am guessing that it means “Out of Town,” because that would explain why she is also listed with her husband and oldest child as Hattie SCHULTE in the second attachment, but it is only a guess. Both censuses show her as married four years with one living child, although her listed ages differ by one year.

It appears that it is a notation for who spoke to the census taker and stands for "other". That is to say, not the heard of the family or the spouse. Most of them read "Wf" for wife.
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Ralph N. Baer        RalphNBaer@...       Washington, DC