If you train your fastText model with skipgrams then your lookup will also get partial words.  So if you have a skipgram of 3 then the word “there” returns [“there”, “the”, “her”, “ere”]… for unknown words, you won’t get the whole word for OOV, but you would get the components…  Now if your word has no components of any words then you’d get nothing, but that’s probably not going to happen if you train on enough data and the input is reasonable (i.e. same language as your train set).