What makes an OSR game an OSR game? Ask different fans of the old-school-revival-movement, and you will get different answers to this. What I noted so far is that a lot of these OSRPG you see on the market have a very prominent “random” factor.
Back in the old days, things were random. You rolled your character up, there was no point-buy-system. You rolled for your background, super powers, mutations and what not. When you had slain the monster, it was not uncommon for the GM to throw some more dice behind the screen to determine your precious loot randomly. And of course, the monster encounter that was the foreplay of the loot-gasm to come all to often was… right, a random encounter the GM had rolled for!
If you want roll stuff up, you need dice and a table (with results, so the piece of furniture known under that name is helpful, too). I bet you already own some dice and if one of those happens to be a d20 and you like to play Mutant Future(tm), please have a look at two of my products:
20 irradiated, weird events for post-apocalyptic campfire nights gives yous (surprise, surprise!) twenty events to throw at your PC at night. A few of these events are monster attacks, but most of them are just weird events with a potential for minor harm.
20 further details for random monster encounters in the wilderness greatly helped me to satisfy my lust for plus-sized product titles AND it provides a MutantLord with twenty additional details for a random monster encounter. The fact that you already had every random monster encounter in the core rules happen twice is no reason to give up on a good and trusty result table if you can bring the spark back by adding some new spices with one additional roll of the dice.
The two are both “pay-what-you-want”, so you can grab them for free, for as little a 10 cents or for whatever you feel they are worth to you. If you want to be on the sure side, download them for free and then, after you were able to evaluate them for your personal needs, buy them again for a price if you found them to be useful. A trade offer cannot get more fair and beneficial for you then that, can it?