A few questions have popped up recently about what the Core Rules are and what Supplement I is. If you play Swords & Wizardry, World of Onn: Supplement I is meant to enhance your experience with that game system with slightly more powerful character options and additional classes, races, spells, equipment, monsters and magic items and presents the core of the World of Onn to use as a backdrop for your Swords & Wizardry games. It widens the Swords & Wizardry experience but doesn't quite make the jump from Swords & Wizardry to Advanced Swords & Wizardry.
The World of Onn: Core Rules is a total rewrite of the Swords & Wizardry: Core Rules and Supplement I that focuses on levels 1-15. Some races (such as Elves and Forged Men) were tweaked and given overhauls to make them more Onnish and less standard fantasy-fare. The addition of Endu, Gnomes and Gnolls as character races, the Spellblade class, some more equipment options, even more new spells that actual players from my campaigns created, a couple of new creatures, more magic items and the addition of a short description of Onn's solar system, calendar and a description of the south-western Lands of Var-Ultar, along with new or improved appendices on simple stronghold rules, optional character rules (which includes a simple skill system that integrates with the base dungeoneering skill system), special weapon and armor materials. There are small nuances to Onn throughout the book as well, but still not in-your-face so the rules can be seperated (mostly) from the setting. I had to bump the font size from 10-point down to 9-point to keep in under 225 pages (my personal limit, the book is 222 pages). Considering S&W 2nd print was 138 pages of gaming goodness and Supplement I was 133 pages I managed to combine both, and still add new material and come in at a page count under both works combined.
If you play S&W and Supplement I do you need the Onn Core Rules? No. Your game will continue to run just fine. There will be a Supplement II for Swords & Wizardry later this year with the new lands and some of the new material that isn't too high-powered or unbalanced from a Supplement I comparison to use in an S&W Onn game.
If you want all the new material then by all means please check out the book. Everything is still S&W and Oldschool (Original, Basic, Expert and Advanced) fantasy role playing games compatible.