Is there any word in English where the sound /o/ stands alone without being part of a diphthong?
OED gives a list of English and American IPA symbols used in the dictionary.
/o/ does not appear as a standalone monophthong; it appears only as /oʊ/ and that is only in the "American" column (for goat). /o/ is not listed in British English at all.
The answer would appear to be No. In standard British and American English, there are no words like /ɡo/, /ko/ where the /o/ stands alone. It remains possible that certain dialects and regional accents in either variety could include /o/, but it would appear reasonable that there is no need to teach it.