The archive contains Millar's extensive research papers on sixteenth and seventeenth century British painting, especially on the work of Anthony Van Dyck (1599-1641) and Peter Lely (1618-1680). The most notable aspect of the collection are the series of journals, with two volumes of indexes, made between 1945-2006 that document Millar's tours of collections of art and related archival sources. There are a large number of photographic files on artists which contain annotated images (the majority are black and white photographs but there are also colour transparencies, cuttings from salesroom catalogues and press cuttings) which are interspersed with related notes, letters and publications. A substantial amount of these relate to Van Dyck and Lely. In addition there are preparatory papers relating to Millar's contribution to the publication Van Dyck : a complete catalogue of the paintings (2004). There are further research papers comprising notes, correspondence and transcripts and copies of original archival sources.
Sir Oliver Nicholas Millar (1923-2007) was an art historian and curator. He served in the Royal Household from 1947 becoming Surveyor of the Queen's Pictures (1972-1988). His specialism was 17th century British painting.