It is not possible for astronauts to reach the Oort cloud with current technology, or any technology expected in the forseeable future.
Recall that it took the Cassini spacecraft eight years to reach Saturn. And Cassini weighed far less than a vessel needed to keep frail humans alive in deep space. Then consider that the Oort cloud is vastly more distant than even Pluto. The Voyagers weighing less than a ton have been travelling for 35 years and are still nowhere near the Oort cloud. How many centuries are you going to confine some poor sod in a capsule for?
As for future technology, we have no evidence that technology more advanced than what we have today is possible. There are no examples of more advanced technology to guide us. It is entirely possible that the advances in technology of last century were an anomaly and will not be repeated.
Fortunately, there is no need to send an astronaut, a being which is astonishingly unsuited to the environment of space. We can send a robot instead, and get the job done just as well, in a fraction of the time, at a fraction of the cost, without the risk.