Andrew Leigh has some tips for Op-ed writers (PDF file). Most of them are sensible, and I’ll add my own, which is that a good Op-Ed piece should contain, on average, 1.5 ideas. That is, the piece should have a main argument that is spelt out and defended in full (there’s an art to doing this in less than 750 words), and an offshoot from the main argument that is stated or hinted at, but not fully developed.
This isn’t a universal rule. A decent piece must have at least one idea (a large proportion of opinion pieces fail these test) and can have two or three. But 1.5 is a good average.