Good question...
It's fun to be a skeptic or to ask important "How do we know this?" type questions. Like many subtle phenomena its existance is inferred and hypothethised from other observations. In this case inferred from the fact that long period comets exist. WTF are they from? How bout a putative cloud outside the Kuiper belt (which IS observed) that would be a good 'Newtonian' fit for the seemingly hyperbolic orbits. I'd put the odds at the Oort cloud's reality as much better than 50%.