Indeed.

While it seems strange that "O" appears as if it's "removed" from total availability for the duration of the OP's simulations/requests, it seems apparent that while "O" is available for other fare bases, it is no longer available for the OP's specific choice of fare basis, due to falling foul of one of the advance purchase and/or min/max stay requirements applicable to his chosen fare. For other fare bases with more lenient advance purchase or min/max stay requirements, "O" remains available.

It's not just enough that O is available; you also have to ensure that changing the date doesn't break any of the other rules of your fare. If a date change breaks those rules, you're going to have to buy up to a more permissive (and more expensive) fare.