The Russian gentiles who moved to Israel, many of whom have a fake State conversion and many who don’t, frequently intermarry R”L with Israeli Jews.
There are plenty of people who go off the OTD, rachmana litzan, even with good Jewish educations, and I’ve never heard any evidence that they’re less likely to marry non-Jews than other secular Jews.
OTDs are far less likely to marry a gentile than secular or Reform/Conservative Jew. The ones intermarrying are far more likely to have left Torah Judaism multiple generations before the person with whom intermarriage r’l occurs.