Oceanic Abstracts - Technical literature pertaining to oceanography, marine and brackish-water environment. [more info]

OnePetro - Broad range of technical literature related to the oil and gas exploration and production industry. [more info]

Oxford Art and Architecture Reference Online - Art and Architecture reference resources [more info]

Oxford English Dictionary Online - A guide to the meaning, history, and pronunciation of over half a million words. [more info]

Oxford Premium Reference Collection - More than 100 reference titles and 50,000 entries from the Oxford Companion series [more info]