Learn English – Different forms of the abbreviations for “identification”

For the first part of your question, I would treat OMO as singular because I'd be referring to a single concept represented by that abbreviation:

• "OMO refers to the buying or selling..."

If you were writing about a number of OMOs, then I'd use this:

• "OMOs refer to the buying or selling..."

For the second part of your question, the first and third options are both correct, but the first is usually more appropriate instances (I also suggest changing "about" to "approximately" {or abbreviate to "approx"})...

In the scenario where I was selling 5 kg. packages of something, I prefer "5 kg. costs approximately €10."

To elaborate further, if I was selling based on weight that the buyer chooses arbitrarily, the businessman in me would prefer something along the lines of "1 kg. costs approximately €3, and for higher quantity orders 5 kg. costs approximately €10."

I commiserate with the desire to avoid a cramped, less readable look.

The citation service that I use, CiteULike, gives a choice of six commonly used citation styles. None show spaces between abbreviated first and middle initials. Otherwise, yes, I think I would add spaces to your remaining examples. So to answer your question, this is what I would do:

A.B. Buffington

and

Vol. 2, No. 6, pp. 195-200