The word "broad" is pronounced /brɔːd/ (some US accents: /brɑːd/) instead of */brəʊd/. The spelling -OA- somehow suggests that these words are closely related and/or were pronounced the same at some point. In simple words, broad rhymes with caught, thought, sought and doesn't rhyme with BOAT, COAT, LOATH, ROAD, FOAM etc.
It seems that boat, road, loath, foam and broad had the same vowel sound /ɑː/ in Old English. All of them were spelled with ā and Wikitionary notes that their pronunciations were with /ɑː/ vowel. Why then is broad not pronounced the same as all those other words?
About a week ago I was reading a book on internet archive (whose name I have forgotten now and I am unable to find it again) which had an entry on "broad" and "abroad". I was lucky enough to take and save a screenshot from the entry. But I do not understand it well.