Museums and Public Art Galleries:
 Ottavio Leoni at the Detroit Institute of Arts, Michigan
Fine Arts Museums of San Francisco
10 works by Ottavio Leoni
Museum of Fine Arts, Boston 
Ottavio Leoni at the National Gallery of Art, Washington D.C.
2 works by Ottavio Leoni
Art Fund for UK Museums
Bowes Museum, County Durham, UK
Medallion portraits of forty celebrated Venetians
Courtauld Institute of Art, London, UK
3 works by or related to the artist
Harvard University Art Museums, Massachusetts
Hunterian Museum and Art Gallery, University of Glasgow, Scotland
Ottavio Leoni in the Los Angeles County Museum of Art Database
Palazzo Ruspoli, Rome (in Italian) 
Ottavio Leoni at the The British Museum, London, UK
Professional Tools:
Artarchiv Sample Artist Symbols + Monograms
Scroll down to monogram 591
Pictures from Image Archives:
Wikimedia Commons Image Database 
Bildindex der Kunst und Architektur (in German)
(a welcome screen is displayed for a couple of seconds before the search results appear)
Articles:
Union List of Artist Names (Getty Museum)
Reference sheet with basic information about the artist and pointers to other references.
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