This is the simplest explanation which is factual, and true!
The common thread that binds the OSR is the hack that created OSRIC and The Basic Fantasy RPG. The hack is that you can take d20 SRD and now the 5e SRD as a starting point for whatever creative vision you possess.
Continued from this initial announcement, that is. 
When the PC reaches zero Hit Points, instead of death they may choose to reduce their Hit Die to the next smaller die size. Only have a d4 Hit Die? To bad, no death save for you.
It should be noted the save is automatic, there is no roll to make. And just to be clear, this does mean, moving forward, the PC rolls this new lower die for all additional levels reached.
There are two now posted on Drivethru.rpg. Ringing endorsements for the product all-in-all. Which is great because I don’t do this for the money. It is out there as a living thing.
Review link: https://www.drivethrurpg.com/m/product/437116
under some sun and 80% humidity on the valley floor. And then you end up here,
After the slog I collapsed in my bunk at Madison Springs Hut and surveyed the map for on the morrow we would assault the summit of Mount Washington.
CC 2 Creature Cache comes a full 7 years after CC1 (PDF is PWYW), the bestiary I used to stock my OSR adventure AA01 Purging Woth N'rld Oakwyn's Muddy Hole. If it is half as intriguing as CC1, the list of 200 monsters boasted between the pages of this new release will be well worth the price of admission. Look, Richard LeBlanc is successful enough he can drive traffic to Lulu. You know he puts out quality stuff if he has that kind of power!
Besides new monsters which every DM needs in spades, LeBlanc is always kind enough, and industrious enough, to include a drawing of each of the creatures. Mandatory, I know, in any monster book, but you don't always get. I like he doesn't shy away from depicting adventures being slaughtered by his monsters. I love that throwback to old school printed materials which usually had one or more pics of some nameless adventurer taking it in the shorts. LeBlanc captures this feel in his drawings and adds to the appeal of the overall package. Black and white, 2-column as god intended.
AA01 Purging Woth N'rld Okwy's Muddy Hole, my first and only dnd module has finally seen the light of day on DriveThru as a physical book!
40 pages, A5 (digest?) size, black and white illustrations and text. Several of the pages in the back are just VTP ads, but I think the book looks cooler with them in than out.
The book is organized with a simple adventure "hook" and a dungeon "ecology" page. Dungeon Ecology is a bird's eye view of how the denizens of the place go about their business. How the different factions interact, and conflicts between the facts of the fables, and the dungeon itself...
One critique leveled at the module is the treasure is not substantial for the levels indicated. Fair enough, each to their own. But I did look at the treasure horde in the module and have increased it here and there from the original edition.
All in all, like many rpg products, this second edition is an improvement over the first. While the content and quirky ideas remain, the organization is better, and therefore should be a more enjoyable product for the DM to run at the table.
Successful delving!
[EDIT/UPDATE] A slightly more polished PDF is available for download now, formatted to make the table useful at the table.
Michael Gibbons goes by the name Aos online and hosts the Metal Earth blog which is filled with gamma-radiated world building. His illustrations are uniquely his own, I commissioned two pieces for DUSRS&S so I like his thick black and white illos. He also released his own comic book, so yeah, another middle-aged ttrpg enthusiast like myself who creates cool shit for their games and also posts them online! If you like Lyn Carter’s spin on the science-fantasy genre you are going to love Metal Earth’s maps and site locations.
He recently released a collection of these creations in a hardback book dubbed B/X Mars. The title immediately clues you in this is another entry in the thick DIY file which is the OSR. And that it is a setting book. Called Mars. So you are in the world of John Carter here, red sands, swords and laser beams, aliens, etc. This is Appendix N stuff. There are 95 pages of content and loaded with Michael’s great drawings. There is a brief, coherent introduction to the Mars of the past and the all-important present followed by character creation. They are Princess, Warrior, Terran, Thark! (always spelled this way), and Menton. Solid John Carter of Mars stuff. Mind Warp is your “magic” system and is the province of Mentons and fields of “mastery” replace skills. Saving throws have their own titles so one is left to assign them as they see fit if you want to compare them to standard OSR saving throw titles. This is how I broke them down; Death Ray/Poison, Warp/Spells, Entangle/Breath Weapon, Fear/Petrify, Rods and Crown is Wands obviously. Desert gear, weapons of the wasteland, airships, weather, these are the first 22 pages.
Layout is simple two-column with a 12/14 size font and bolded section titles, making it an easy read. There is a generous amount of white space (the book is 11 x 8.5) and I, for one, like this in my game books because I write on them. The way POD page count falls you end up with 5 blank page faces at the back. I could run a campaign with all my notes and prep written in the book with this kind of writing space. I think this is the only setting book I’ve picked up which has offered up this amount of white space. I don’t think this is overly intentional. I think Gibbons is like myself, an ametaur layout guy so each book looks loose.
Part three and part four of the book is your gazetteer. Like McKinney’s Carcosa setting book, B/X Mars is content to give a detailed description of only a small section of the world. In this case it is a land on Mars referred to as “Zerzura”. The map of Zerzura is a good example of an “evocative” map. The hand-drawn cartography and unique place names do what the best ttrpg maps do, spark adventure ideas! Gibbons has several area maps on his blog you could use for whole campaign maps, all of them he draws spark adventure ideas just looking at them. A bestiary of unique monsters to the setting and a couple of detailed site locations finishes the book up, and like I said earlier, plenty of content you could run your 1st edition Gamma World campaign for years just building off the stuff in B/X Mars.
What it doesn’t have is charts. Specifically random encounter charts. Look, I know, creating tables with your publishing software can be a whole hell of a pain in the ass. Being able to type away endlessly and shape that up into readable content is easy. You will get your book done a whole lot faster if you skip tables, I get it. But OSR begs for random encounter tables. This harkens back to the game’s roots where the next exciting adventure is unknown till encountered by a chance roll of the dice. And is a big omission for an old-school setting book. I am going to correct this by creating random encounter tables for the game and post them below, free, for the world to use. Now, grab hold fast the line and swing from your armed airship into fantasy last-planet adventure with B/X Mars!