INTRO/RANT
There seems to be a lot of confusion coming from newer players regarding rolling affixes on their equips. I know you veteran pro players think that we're just lazy to be bothered to read the docs, but honestly, the mechanics of median are vast, broad, and deep. It can be difficult to know where to look or what to look for. And even after reading it in the docs, it can still be confusing.
From the perspective of a player raised on the typical console rpg, where you don't need to know anything about the mechanics, I can personally say the adjustment to median was a difficult one. It's vastly different than the games I'm used to. I found that it's a fantastic mod, though, once I started to get the hang of things.
My apologies for the rant, but I think that the veteran players sometimes take for granted how vast the content must seem to a first time player. As someone still learning the game, it's still kind of fresh in my memory how bewildering it was. Anywho...on to the guide (well, more of mini guide, really).
That being said, I am still learning, so if I make any mistakes here, I'm depending on you veterans to point it out, so I can correct it.
Rolling Your Oskill Miniguide (or item affixes, generally)
Hey, oskills! Cool! I want one of my own! I want to play the Lazydin, but I just can't seem to roll Ring of Light! I want to build my very own tantrumsin, tantrumdin, or tantrum barb, but tantrum just won't show up! What do I do?! Agh!!



Important Numbers
Ok, so first off, there are a few numbers you need to know: item level (ilvl), quality level (qlvl), and affix level (alvl).
Item level, hereafter referred to as ilvl, is determined by by area/monster lvl where it dropped. Or character level (clvl) if it was gambled. It can be viewed by altering the settings in plugy, iirc.

Quality level (qlvl) is determined by the base item and tier level (1-6, sacred). Qlvl, along with other useful things (like max sockets, min max dmg, etc.), can be viewed here: base items.

To find the affix level of your item, you need the ilvl and qlvl and plug it in to the calculator here: prefixes/sufixes. It's the first one from the top.

An Example
So let's tie it all together with an example:
I want to roll Ring of Light on so I can make progress with my lazydin build. Let's assume I picked up a rare rondache, tier 5, with an ilvl of 83.

It's qlvl is 65. Punch it in to the calc and u get an affix level of 67.


As can be seen, above, the required affix level for ring of light to appear is 72. If your items affix level is lower than the required alvl of the affix, then it won't roll. So you won't be seeing RoL on this shield.
At ilvl 99, the affix level is 99 regardless of qlvl, so you could try rolling with a ilvl 99 shield as another strategy.
Rareness of Affixes (frequency)
You might also find that even if your item meets the alvl requirement, that it seems like it's still not rolling. This is where the number listed under frequency comes in. iirc, I heard Rishab mention that the lower the frequency number, the more rare the affix is.

If we look at Ring of light, it's frequency number is 25. Compare this with a much more common affix--Enhanced Defense +(301 to 350)%, which has a freq of 4072.

Clearly, there's a big difference here. But keep at it. So long as you meet the alvl with the right item type, you'll get it eventually.
Groups
There's also a column labeled "group" with a bunch of numbers under it. iirc, i heard it mentioned that only one affix from a particular "group" will roll on an item at a time. oskills, for instance, all seem to be under group 217, meaning that you can only roll one oskill per item.

Other Tips
If your affix level is 48, you could try rolling with the lowest possible affix level on your item that will give you that affix. This will reduce the pool of potential affixes on your item, possibly improving the chance that u'll get what you want.
List of some notable oskills
For a list of notable oskills, check out my update of the oskill list--->>>Oskill List by nealn
Anyhow, gl hf!