And in Mike's post above it:
"1. Public domain - do whatever your like, no credit required or even expected, no responsibility assumed."
IANAL but I stand firm by my words: Public Domain means "Do whatever you like with the binaries and sources that henceforth go open for everybody and for any purpose, public or private, free or commercial, legal or illegal. Attribution to the original author is not required (but will be appreciated if given)."
I used to do quite a bit of my own research regarding permissive licenses, and from my perspective, based on the former Public Domain status of O2, Charles (or any other person, for that matter) is in their indispensable right to change the license of their copy of O2 sources to any other license -- permissive (BSD, MIT, or similar), restrictive (GPL, LGPL, or commercial) -- or even close it (the copy) with its modifications, fixes, etc., if any, completely
at any time starting from a certain date. Other people's copies and/or mods based on the sources
earlier than that date still remain in the Public Domain (if those people so wish).
It means that stricter license terms (and MIT is clearly stricter than Public Domain because it requires attribution) may not be imposed in retrospective. Which means Charles may not take back his words spoken in the thread cited by Jose.
OTOH if I were in Charles' shoes, I'd probably leave the FB-based O2 sources in the Public Domain altogether for historical reasons but I'd put the self-compiling Oxygen's sources under the terms of MIT license. Just because a self compiling compiler of such quality, versatility and completeness is a no-nonsense achievement, and the name of its creator should not go into oblivion by all means.
If O2 isn't going the open source route with a license to state its status, I no longer have interest in the gift.
John, "open source" is just what it literally says: "source open to all" -- subject to certain conditions, if any. Do not try to force us to see things that aren't there. You know, Occam's razor and stuff...
Public Domain presupposes that everyone who cares has always been, still is, and will continue to be, granted free and unrestricted access to O2's source code at least in its FreeBASIC notation. So
it is open source regardless of your earlier, current, or future interest in the project.