That English contains monophthong [o] words (such as the traditional realization in certain dialects or broad transcriptions like GOAT before certain glides/environments, or in loanwords and narrow phonetic descriptions) is a direct everyday observation of English phonology that requires no citation.
The claim is a straightforward linguistic fact about English phonology (the existence of the monophthong [o] or close variants in various English phoneme inventories or phonetic realizations), making it common knowledge by direct observation of phonetic systems. None of the retrieved papers specifically test or refute the existence of [o] monophthongs in English as their core thesis, so no indices are placed.