IF you get your contract signed before the open house, the realtor there will have some embarrassing answers to give. All any other buyer could do then is to make a backup offer, whereby they are in second place. Only if you could not complete the sale, would they be able to buy it.
If your contract is not signed before the open house, then there is a greater problem for all parties. As an agent in that position, I would feel a need to explain to anyone interested that we have an accepted but not signed offer and that if they still wanted to buy it (at a higher price for instance), I could not guarantee that their offer would be accepted except as a backup.
But, other than pushing to get it signed right away, I would unlax a bit. It's been said often that open houses are to make the owner feel that their agent is really working for them than to actually get a sale, meaning sales are not often made as a result. And if when that happens, folks usually think about it a bit, see it a second time.... then make an offer.