As the title says: I found some NOS Osram SOX lamps for very cheap on eBay (I got one 18W, two 55W, and two 90W). The 55s and 90s were bundled as a lot for $100, but I put in an offer for $85 and the seller accepted. I got the 18W for $24 (original price was $28, I offered $24) + $10 shipping.

Now, the question is, what ballast should I purchase for the respective lamps? I know I can get some 90W ballasts from Francis through eBay, but what about every other common wattage? I've heard of some people using some kind of Fullham Workhorse ballast, but which kind, and for what lamps etc. Sorry, I'm pretty new to the land of monochrome...

Your best bet is to get a fluorescent ballast to run it. I’m sure WorldwideHIDCollectorUSA will help with how to wire it and what wattage of ballast to use as he’s the expert.

If you want to go the magnetic ballast route, I have used a 277V F20T12 preheat fluorescent series choke ballast and a FS4 starter along with a 120-277V step up transformer to run 18W SOX low pressure sodium lamps before beacuse although 18W SOX low pressure sodium lamps have a similar arc voltage drop and operating current compared to F20T12 fluorescent tubes, I was not able to find a way to get them to run on 120V F20T12 preheat fluorescent choke ballasts. After seeing several European members using 220-240V F18T8/F20T12 preheat fluorescent choke ballasts and 4-65W starters, I decided on the 277V F20T12 preheat ballast setup with a step up transformer and FS4 starter and it seems to work decently well.

Additionally, I have also gotten a 90W SOX low pressure sodium lamp to run off of a 70W M98 pulse start metal halide HX autotransformer ballast using a European Philips SX76 2 wire parallel ignitor in place of the original ignitor.

I am also pretty sure that a 50W M110 pulse start metal halide HX autotransformer ballast with a European Philips SX72 ignitor may also work well but have not tested that setup yet.

I also have used a 35W M130 pulse start metal halide CWA ballast to run a 35W SOX low pressure sodium lamp before but used a European Venture Lighting SOX low pressure sodium ignitor to help the lamp start reliably. It was somewhat underdriven but ran pretty close to its ideal current.

Hey, I run my 55w SOX lamp with a Fullham WH5-120-L ballast with 2 red leads and 1 yellow lead. Works fine and I've had no issues. Don't have any info on 90w and 18w though. Hope this is helpful!

Thank you very much!  To which points on the socket do you wire the three wires?

In this case, red is hot and yellow is common. The reds to one terminal, and the yellows to the other.