In possible defence of the Otel website, what can happen with large booking agencies (and often does, especially with flights), is that, in the time it takes to fill out the necessary personal information, credit card details etc, the one room that may have been available is taken by someone else. When you consider how many people may be trying to book a room (just like you) and someone else may be just seconds ahead of you.
I had the same problem once, booking flights over the telephone with a travel agent. There were 5 seats available at the beginning of our conversation and by the time she actually clicked on her computer to book them, there were only 2 left, fortunately we only needed two.
I thought this may be worth mentioning because Otel.com may be excellent so give them the benefit of the doubt.