Alright, people really seem to love OSE as an alternative to B/X and other early versions of the game and retroclones. If you are one of those people, tell me what I should get OSE versus using my Rules Cyclopedia or my old B/X books? Also, tell me about the much lauded support for OSE, especially support in the weird fantasy/genre mashup space, if it exists.
Note that this isn't going to be the hardest sell in the world. I have some extra Christmas money, but I want to know why folks love it so much.
To bring things back around a bit...
OSE is as direct a retroclone as is legally possible to make of B/X. It has no content from the BECMI line or the RC, as far as I know. As mentioned, the books are new and designed in a great, easy to read layout. A few minor things that were inconsistent between B and X were smoothed out and they include charts for using either old-style descending AC or the newer ascending AC. They also make great use of space by printing on the end papers. Usually the charts and tables you'd have with a DM's screen.
There are a lot of adventures and 3PP writing for OSE now. It's kinda staggering for a "small" retroclone. Even a few award-winning adventures. Dolmenwood is great, except a few bits that have already been mentioned. Halls of the Blood King is a great adventure. The Incandescent Grottoes is another great one. As is The Isle of the Plangent Mage. The thing I love about OSR games, especially OSE, is the level of weird that goes into it. I really miss that. As mentioned, Planar Compass is their Planescape / Spelljammer mashup.
If you want pure B/X, pick up the Classic Fantasy black books. The Rules Tome is all you need as it contains all the rules for the game. The Player's Rules Tome is handy to pass around the table and only includes the player-facing rules, so no monsters or treasures, etc.
They recently did Advanced Fantasy. Which takes the races, classes, spells, etc from AD&D and fits them to the B/X rules. You can have either race as class per B/X or have race and class separate per AD&D. It's important to note you'll need both the Player's Tome and the Referee's Tome as the full contents are split between the two books, akin to the PHB and DMG.
There are some quibbles about the conversion of AD&D stuff to the Advanced Fantasy rules. Like the paladin's stats. In AD&D the paladin needs a 17 CHA, but in Advanced Fantasy only needing a 9 CHA. As far as I can tell there's a few oddities like that, but you're not dealing with wildly different abilities between AD&D classes/races and Advanced Fantasy classes/race. A paladin's a paladin and a barbarian's a barbarian.
ETA: To be clear. Advanced Fantasy has everything Classic Fantasy does plus their conversions of AD&D stuff into the B/X rules.