The Internet Archive has digitized some of Osborn's publications. Click on the following links to see more:

• The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, 1929, Volume 1
• The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, 1929, Volume 2
• Proboscidea, 1936, Volume 1
• Proboscidea, 1936, Volume 2

Other Osborn publications can be searched for through the Internet Archive here.