Old School Magic comes at 29 pages, and is about new classes, new spells, and options / suggestions for the GM to determine how magic works in a campaign.
-- MAGIC OPTIONS: The first part discusses low, medium and high magic settings, and provides magical systems (namely: incantations, mana, and star magic). Note that these magic systems are not necessarily exclusive of each others, and could probably be combined in some way. For example, a GM might houserule that in his campaign humans could only cast incantations, while elves and gnomes being fey-related use the mana option. There is one thing I found strange, wondering if it is a typo, but in any case being easy to change/houserule: about spells that become incantations, the text states: "Replace the normal casting time with one week per spell level". Personally, I would rather go with one turn per spell level, which is long enough IMO.
-- NEW CLASSES: The second part is about new character classes. I admit that I am a sucker for new character classes. This book includes the Alchemist, Artificer, Conjurer, Elementalist, Hermit, Holy Man, Naturalist, Sage, and Seer. All of these classes fit perfectly along 1e classes in terms of game mechanics and design: they are clear and simple, with the kind of abilities you could expect from 1e rules. Overall, I am not sure however, I would like to play one of these classes instead of a regular mage, illusionist, cleric or druid (where I would be ready to play an old-school psionicist by the same author). Yet, the GM could allow these classes to races that normally don't have spellcasters, such as allowing dwarves alchemists and artificers, as well as halflings holy-men and naturalists. This would make things better, especially if level cap is above 10th level. The author doesn't say much on this subject (apart that dwarves can be elementalists up to the 5th level), so it's easy to implement at leisure by the individual GM. Other than that, I am dubious about some classes' names: Naturalist as a replacement of druid in a low magic setting? I rather see this class as a Woodsman or Wilderness Hunter, not a religious type. Then, if you use Holy Man in a setting featuring all regular classes, a change of name might be necessary (such as Zealot, or what not), as a "holy" person who doesn't cast spells seems a little strange. As for Conjurer, I regret that their Summoning spells' duration is not extended.
-- SPELLS: The third part describes 31 new spells. Most of them seem balanced and usable, but I have some doubts about: Iron Warrior (seems overpowered in summoning an iron golem for 1 turn pr level).
-- THINGS THAT ARE LACKING: 1) There is no table of contents at the beginning of the book, and it wouldn't hurt to add one. 2) Ley lines: the author mentions the use of ley-lines a couple of times, but there is no rules about them in this book (apart an extremely vague suggestion); maybe he wrote something about this into another supplement, but this is neither indicated.
-- LAYOUT AND ART:: as much as I like the book's content, the art and layout is rather bland and uninspiring. If, someday, the author was to compile all of his old school (1e) contributions into a single book (I can dream after all), I wish he would come with something far better in this regard.
-- CONCLUSION: Old School Magic is a good product for Osric / 1e, and I recommend it.