As of Xbox One, the OS version format has changed to the buildtag that has been used for Windows since 2001.
Buildtags can come in two formats, they are almost identical.
An example of the first format of buildtag is: 6.2.10210.0 (xb_rel_launch.131118-1159)
An example of the second format of buildtag is: 6.2.10210.0.xb_rel_launch.131118-1159
The buildtag is in the format major.minor.build.delta.lab.date-time, where:
major is the major version, minor is the minor version, and build is the build number.
For Xbox One, all currently known OSes have major version 6 and minor version 2. Therefore Xbox One's OS is based from Windows 8, as this uses the same major and minor version.
Delta is an official MS term. It is basically a minor build number showing the number of times the build has been recompiled: for Xbox One there has been no known usage of the delta which is always 0.
Lab is the lab in which the build was compiled; as in, which MS team compiled this build. All known Xbox One builds have labs that start with "xb_".
Date is the date when the build was compiled, in yymmdd format; Time is the time the build was compiled, in 24-hour format.
So, putting this all together for the above build:
Xbox One OS build 10210.0 was compiled by xb_rel_launch on 18th November, 2013, at 11:59 am.