Doing this as a comment, not answer, because this is not really canonical.
Splunk is only very loosely coupled to the OS and upgrades of the OS are not particularly important to Splunk. If there's no clustering in your environment, then you can do whatever, IMO, with the caveat that you probably really want all the OSes to be of nearly the same version. (If for no other reason than management should be easier).
With indexer clusters (and perhaps search head clustering) you'll want those boxes - the CM and indexers, or whatever is involved with SHC, to be upgraded all at once or at least within a relatively short time. Of course, to upgrade an indexer cluster, maintenance mode and all that needs to be done just because the expected downtime will likely be long enough you don't want panic bucket fixings...
http://docs.splunk.com/Documentation/Splunk/7.1.2/Indexer/Upgradeacluster
Otherwise, it really shouldn't be a big deal.
