The Mesosphere DCOS is built around the Apache Mesos kernel
The Mesos kernel was developed at UC Berkeley in 2009 [1].
Spark was written as a sample app on top of it [2].
Ben Hindman and his colleagues at the UC Berkeley AmpLab had always envisioned Mesos as a kernel inside of a full-blown operating system [3]. They finally brought it to market.
[1] https://www.usenix.org/legacy/event/nsdi11/tech/full_papers/...
[2] "We have implemented Mesos in 10,000 lines of C++. The system scales to 50,000 (emulated) nodes and uses ZooKeeper for fault tolerance. To evaluate Mesos, we have ported three cluster computing systems to run over it: Hadoop, MPI, and the Torque batch scheduler. To validate our hypothesis that specialized frameworks provide value over general ones, we have also built a new framework on top of Mesos called Spark, optimized for iterative jobs where a dataset is reused in many parallel operations, and shown that Spark can outperform Hadoop by 10x in iterative machine learning workloads." ibid.
[3] http://people.csail.mit.edu/matei/papers/2011/hotcloud_datac...
In the Mesosphere world, Kubernetes is a "datacenter service" which is installed on your datacenter so that you can run Kubernetes workloads. You might also want to install DEIS to run DEIS-organized workloads. Or Spark, for Spark workloads... and so on -- all multitenant in the cluster. This is what the DCOS is uniquely good at, and why it qualifies as a true operating system.
After reading the title I was a bit tempted to call BS since I think Kubernetes should have the rights to "The first Datacenter OS" tagline[1]. However, after reading up on the project details at https://mesosphere.com/learn I can see how Mesosphere came to the conclusion of being a DCOS, if not necessarily the first. Mesosphere goes a bit further than Kubernetes and offers a solution to the storage problem and attempts to address other "userland" concerns by shipping Apache Spark, Cassandra, Kafka, and Hadoop. So maybe it would be more accurate to call this a datacenter distro on top of the Kubernetes kernel?
Regardless, I think the concept of a datacenter OS will be the key to commoditizing IaaS providers and leveling the playing field in terms of features and usability for those who have not given up on the dream of running a "private" cloud.
Why will the DCOS work where others have failed?
Current solutions aimed at taming the datacenter operate at the machine/VM level, which exposes the OS for each machine, and completely punts on the application. Guess who gets to stitch it all back together? A DCOS is designed to manage applications directly, commonly via application containers, which means we can treat the OS running on the underlying machines like firmware and limit our interactions to basic updates and minimal configuration -- think CoreOS.
What about PaaS?
That's a topic worthy of a lengthy discussion, but I think it boils down to the lack of control found in most PaaS platforms[2]. In order for a PaaS offering to be successful it must make opinionated decisions about how to deploy and run applications; a bit too inflexible for most people. On the other hand, a DCOS seems to hit the sweet spot between IaaS and PaaS.
[1] I'm sure you can make an argument for Joyent's SmartDataCenter (https://www.joyent.com/private-cloud) as well. [2] Deis (http://deis.io/overview) attempts to address this issue.