So, as we all know, Data's storage capacity is 800 quadrillion bits, or ~88 pebibytes. I'm curious how much space the OS for Data uses. Is there anything anywhere that states this?
Star Trek writers are not, by and large, computer scientists or software engineers, and neither, they know, are all of their audience. As such, details such as operating systems, or indeed even basic operating principles, are rarely described in any detail. We're told, for example, that Data has a "positronic matrix", but we're never really told what this means, any more than Asimov, who first used the concept of positronic brains in his robot novels, ever really told us what it means.
Other similar buzzwords have been bandied about to describe Starfleet's futuristic equivalent of our real-world electronic systems -- duotronic, isolinear, etc. The principles behind them have never really been discussed, with the sole exception, perhaps, of "The Ultimate Computer", where we learn that part of what makes M5 special is that its personality (to the degree it has one) is derived from the memory engrams of its creator, Dr. Richard Daystrom.
(The engram, by the way, is not something Star Trek made up, although it is so far only hypothetical as a unit or location of memory storage in a human brain).
Generally, such buzzwords and explanations come to pass because they're important to a story -- either directly to a plot, or to something about the ambiance or world-building of a plot. "Duotronic" for example showed up for the first time in "The Ultimate Computer". The question of how much space The Doctor's program and memories consumed became integral to the plot of a Voyager episode, in part because some writer actually thought through the implications of an ostensibly emergency-only AI running continually for years without reset.
But they never did a story in which Data was in similar danger of running out of memory, and so the question of how much of that memory is consumed by his basic operating software has never been canonically answered.