This is a tricky one to review - I really like RPGPundit as a, well, as a pundit, and subscribe to his YouTube channel. I thought Lion and Dragon was fantastic, a really good re-imagining of old school D&D in a genuine medieval setting. The L&D magic system is really great.
Old School Companion is a mixed bag. It's basically a collection of stuff from the RPGPundit's regular "...Presents" series of PDF products. I can't quite make up my mind if the content if this book is basically inessential, or if it just doesn't really fit with my personal DM style/game requirements.
There are some new classes and crit tables - these are decent, especially the Archer class as a variant of the original Fighter. Some of the info about the clerical order is also useful, even if like me you use Catholic Christianity as the basis for your game's religion rather than L&D's "official" Sol Invictus cult.
However, a lot of the content of the book seems, while interesting, not really a lot of use. There's a lot of material about real world magic like the Goetia and tarot and so on. This is all correct (the Pundit knows his occult) but I can't really see it being much use in game, the original L&D magic was pretty comprehensive and if you wanted to know about the Goetic demons there are plenty of places online you can print off a list of them. Similarly with the tarot - i can't imagine having a fully detailed tarot session in game, you'd just hit the high points that were relevent to the adventure and crack on. But then again, maybe that's just me. I know about the occult as well, so for someone without that background all this might be gold.
I have a similar problem with some of the mundane stuff towards the back of the book about costs of housing, travel times and so on. In my games, this kind of stuff is never really that important and while I expect it's well researched it just doesn't really seem to justify the page count to me. I am aware though, that as an Englishman, a lot of this "medieval authentic" stuff is just stuff I know from being English and growing up in England and having an interest in history. For an American reader, for example, this sort of information could be exactly what they need.
So there you go. The book is nicely designed and put together (although not as nicely as the original L&D book which was something special), but what you get out of it is probably dependent on your baseline knowledge of history and the occult, and how much crunch you like in your games. But the Pundit is still a top chap so I'm still giving it 4 stars.