The Oort Cloud is a theoretical cloud of icy objects that is believed to surround the outermost regions of the Solar System. While its existence has not been directly observed, it is widely accepted among astronomers and scientists as a possible reservoir of comets and other icy bodies.
The Oort Cloud is thought to extend from around 2,000 astronomical units (AU) to as far as 100,000 AU from the Sun. To put this in perspective, 1 AU is the average distance from the Earth to the Sun, which is about 93 million miles (150 million kilometers). The outer boundary of the Oort Cloud is estimated to be roughly halfway to the nearest star system, Proxima Centauri.
As for your question, the Oort Cloud would not act as a failsafe measure to prevent the dilution of space medium inside our Solar System with the space medium outside its boundaries. The Oort Cloud primarily consists of icy bodies such as comets and does not contain a significant amount of space medium that could mix with the surrounding interstellar medium.
The interstellar medium is the sparse gas and dust that exists in the vast regions between stars in a galaxy. It is different from the space medium within our Solar System, which is primarily composed of the solar wind and the particles and debris within the planetary systems.
The Oort Cloud's main significance is related to its role in providing a potential source of long-period comets, which occasionally get perturbed and fall toward the inner regions of the Solar System. These comets can then become visible from Earth as they approach the Sun.
To summarize, while the Oort Cloud is an intriguing region in the outer Solar System, it would not serve as a failsafe measure against the dilution of space medium within our system with the interstellar medium outside its boundaries.