One of the biggest goals of the main products of the Old-School Renaissance, I think, is the entire philosophy of a “toolkit” or “DIY” philosophy. We see it in Labyrinth Lord and Swords & Wizardry, and we see it in the contributors of Fight On!. By design, the simulacra of the old school do not cover every situation in terms of rules and mechanical resolution; adjudication by the DM/GM/Referee is key. Similarly, the games are built for your own additions as you see fit, be they a new thief class, optional poison rules, or throwing in some new magic items or effects.
I think one of the best and brightest examples of this philosophy in action has to be The S&W Companion. Designed to support Swords & Wizardry, this resource page is a tremendous example of the sort of fun additions you can add onto a game. I was surprised just how fast this collection has grown, and hope it continues to do so. Adventures, combat tweaks, and new classes all await your viewing pleasure!
(I especially like the add-on classes, from the Druid to Assassin. Very nice write-ups!).
Perhaps one of the nicest things about dipping your toe into the world of old school is that if you don’t feel you are that creative, have a case of the ol’ writer’s block, or don’t want to mess with optimizing your game of choice for your table, there are some of the most talented gamers in the world who have already provided a ton of cool stuff for free. The S&W Companion is a splendid instance of that.