Thanks for the information!
Some interesting copyright information pertaining to various works:
http://copyright.cornell.edu/resources/publicdomain.cfm
The section "Works First Published Outside the U.S. by Foreign Nationals or U.S. Citizens Living Abroad" indicates that many works first printed outside the U.S. prior to 1923 are in the public domain (there are some special circumstances listed). Since Oscar died in 1900 that would tend to suggest that any of his works published during his lifetime are in the public domain.
I guess they may be applying the section "never Published, never Registered Works" for quotes, which are verbal occurrences. This section grants copyright for the life of the author + 70 years or 120 years after the creation date.
However, this brings up some interesting questions:
1. What if Oscar printed, or hand wrote, one of his quotes inside a copy of one of his works, such as on a introductory page. That would tend to suggest publication for that quote and thus it would be in the public domain under the pre-1923 scenario mentioned above.
2. A newspaper published one of Oscar's quotes during his lifetime. That would also suggest a publication date prior to 1923 since they would likely have a right to do so under fair use - i.e., as a news story.
Disclaimer: I am not a lawyer - this is purely a quick brainstorming session on this matter.