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."
English is not unambiguous, so there is no absolute rule. However, generally speaking the last noun is modified by the preceding nouns. "Sofa box container", has container as the last word, so it refers to a container, and the type of container is one for a sofa box.
When you hyphenate the meaning doesn't change much, so, from your example:
Queen-killer — a queen who is also a killer
Is not correct, a "queen-killer" is a person who kills queens. The hyphen just tightens the relationship.
The emphasis is the same with the genitive but the relationship is a little more ambiguous:
Queen's killer
Can mean a lot of things, the two most obvious candidates being a killer who works for the queen or a person who killed the queen. Nonetheless, the rule still applies, it is a killer, and the type of killer is "queen's".
So the bottom line is that usually the last word is the main word, the rest are modifiers.
For your example, database containing machines would be appropriate, since the main subject is the machine, and the rest say what type of machine. In this particular instance though the idiom would be just plain database machines, or database servers.
Best Answer
The word ox comes from the Old English oxa. In Old English, as in Indo-European languages in general (historically and even today), the number of a noun (singular or plural) and its function in a sentence—whether it was the subject, direct object, indirect object, or had some other relation to a verb or another noun—was largely (not solely) governed by sets of endings tacked onto it, or changes made to the vowels in it. These sets of endings or changes were called declensions, and each type of relationship associated with an ending is called a case.
There were a number of declensions in Old English; the two most prominent were the weak declension, containing the weak nouns, and the strong declension, containing the strong nouns. Old English oxa was a weak noun. The forms that we have of its descendant today are derived from the nominative case endings; these are the forms that would indicate that a noun is the subject of a sentence, or the forms that would be used when writing a list of nouns.
Since oxa was a weak noun, its plural form (the nominative plural form) was oxan. Over the course of centuries, the a "weakened" to an e, giving us oxen.
Fox, on the other hand, comes from the Old English fox, which was a strong noun; its Old English plural was foxas, whence we get foxes.
The source I used to confirm the declension of fox has an entry for the Old English box; however, it has no declension information. Using this translator, however, it appears that the nominative plural was boxas, giving us boxes.
Although Modern English has largely dropped the declensional suffixes we got from Old English, we occasionally see them peeking through, as we do here.