Electronic Monographs series from the British Institute at Ankara.
Opuscula : Short Texts of the Middle Ages and Renaissance (is also mirrored at the older opuscula.synergiesprairies.ca address)
Opuscula (journal of the Swedish Institutes at Athens and Rome, has a one-issue paywall on articles)
Opuscula Philolichenum (The New York Botanical Garden, journal of short papers on lichens and lichenology)
Opuscula Zoologica (Supported by the Hungarian Academy of Sciences, hydrobiology and soil zoology with a focus on Central and Eastern Europe)