If you read the page you linked more carefully, you will see that OSI does not own a valid trademark for "Open Source", only for "Open Source Initiative".
OSI in fact tried to file for a trademark on 'Open Source' in 1999 [1], but failed because the term is 'too descriptive'.
Edit: I doubt the license for this code is an OSI-approved one with this registration business. But I have not checked and I won't check.