This paper explores how phraseological units compete with monolexical units in the monumental Historical Thesaurus of the Oxford English Dictionary (HTOED), primarily in its online version. By means of several case studies, the status and coverage of phraseological units is analysed both in the synonym lists (monolexical and phraseological synonymy) and in the lower-level category headings (lexicographic synonymy), where paraphrases and definitions are considered as further multi-word expressions.