While it's known to raise AST/ALT/LDL and lower HDL, it's not clear in the medical literature that this qualifies as "hepatotoxic". Some studies measure liver enzyme activity more directly while administering oxandrolone and while those studies see these changes in "liver values", they don't see evidence that that the liver function is actually impaired.
I think there's strong evidence that there's additional stress on the liver (more work for it to do) but not lots of evidence that most people actually experience hepatotoxicity (that it's actually doing any less work than before). Some % of people have been seen with liver damage/failure/cancer caused by oxandrolone but it seems to be rare, as the FDA still considers it a safe and effective drug. These severe complications do seem to be at least somewhat related to how long oxandrolone is used continuously (shows up mostly, but still rarely, in long-term continuous use).
Note that just lifting heavy weights without AAS also raises ALT/AST, but this generally doesn't mean the liver is stressed out by the weightlifting. ALT/AST is an indicator that something is potentially wrong, but not an indicator that something is definitely wrong. If you come off anavar and drop to a cruise dose of test/whatever, and take a few days off lifting to rest, and your AST/ALT is still elevated, then yeah probably something is wrong with your liver because there's nothing else that would be causing that elevated AST/ALT except for a damaged liver.