Green’s Dictionary of Slang

1. menstruating.

1967–8		Baker et al. CUSS 164: O.T.R. / On the rag / Constantly complaining and irritable. Be menstruating.
1971		E.E. Landy Underground Dict. (1972).

2. thus (of either sex) irritable, in a bad mood, tetchy.

1967–8		Baker et al. CUSS 164: O.T.R. / On the rag / Constantly complaining and irritable. Be menstruating.
1981		Eble Campus Sl. Mar. 5: OTR – [...] snappish, testy, in a bad mood.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 If you know that O.T.R. means ‘on the rag’ which means ‘having a menstrual period’ you are not in the clear water yet. ‘Jack’s been O.T.R. for the past week.’ does not mean that Jack has had his period but that he has been in a very bad mood.