In defining OTD I suggest that one line is in total non-observance of Shabbos (including yuntufs) and Kashrus. To be on the “other” (assimilated, non-frum) side of the line, one must be working at one’s job on Shabbos and in no way making “Shabbos” special (e.g. if you make a festive meal on Friday in honor of Shabbos, you aren’t totally off the derekh even if you go to work on Saturday morning). Not fasting on Yom Kippur and not going to hear Shofar on Rosh Ha-Shana, and not going to some sort of seder on Pesach probably indicate you are “gone”, but if you take off on Yom Kippur, have matsa on Pesach and shake a lulav now and then on Sukkos, you are still not totally OTD.
This suggest that to study the problem, one needs to have a scale reflecting both halacha and how various halachot are understood socially (e.g. many totally un-frum people still make a special meal on Shabbos, or take off on Yom Kippur, or attend a seder), and one needs to ask if someone is truely OTD if they have gone from frum to some level at which they while be strictly not “orthodox”, are still acting in ways that are clearly unassimilated.