The Yubikey output contains the serial number, an OTP, an incrementing counter, and possibly some other things that I have forgotten. It is not time limited, so you could generate (say) 15 OTPs in a row from an offline computer, and record them on a bit of paper for later use, as long as they were used sequentially. This would work, but it would be tedious to type in every time.
Got it, the counter. Thanks.
The other thing is that you can skip OTPs if you want to, because of that counter. Therefore, you could generate a bunch of keys, but as soon as you used key #15 from the example above, all the previous ones would become invalid unless you had used them in sequence.