Orion is a short, 15 to 20 minute HL mod, released in 2004. Like so many other mods, it stars another anonymous Black Mesa worker who just happens to be wearing an HEV suit the day of the Resonance Cascade. Is it meant to be Freeman? Who knows...
The mod is short, but it has some solid mapping and the action is quite fun. There's quite a lot of variety in terms of where you play. You get to visit the industrial areas, office areas and a couple of outdoor spots. The industrial theme is by far the most common one and I'd have liked to see more of the others. Like in Half-Life, you get the feeling Black Mesa is falling apart and falling into the hands of Xen. This theme kind of dies off towards the end however, which is a shame. As for actual problems in the mod: there's some intro text that gets shown every time you reload the first map. Quite annoying to have text blocking your view when you're trying to kill that alien. The level transitions are crude, but they at least work. There is also one spot where you can get stuck on a ladder, requiring a reload.
There are no custom models/textures but, unusually, it comes with a custom HUD. It's a cartoony sort which you may or may not like. Personally, I find it very appealing. It's a nice touch to this little mod.
Overall it's a good 15 minute blast well worth your play. The mapper included the RMF files too so you can have a look around and see how everything was done.
In the game the title is referred to as "Orion: Part 1". As far to my knowledge, a part 2 was never made.