Learn English – Is “Cost savings” a singular term or plural terms

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."

Cost is typically left without the s if there is only one purchase:

The cost of thread

Using costs here would imply more than one type of cost:

The costs of homeschooling

You can still say this without the s but the connotation shifts back to one solitary cost:

The cost of homeschooling

It is ambiguous whether these costs are monetary or emotional or something else. Referring to bundles of purchases will typically use costs:

Needle costs are rising

These are my needle costs

If you used cost here it would shift back into a description of needle markets or a single project's purchase:

Needle cost is rising

This my needle cost

The differences between cost and costs in these examples are subtle and could be entirely regional. One final note is referring to how much something costs. This will follow standard pluralization rules:

The book costs $4

The books cost $4

To directly answer your question, I would use costs: Sewing Material Costs. Variations:

Sewing Material Costs — the money spent on sewing material over multiple purchases

Sewing Materials Cost — the amount required to buy "sewing materials"

Sewing Materials Costs — the amount spend on sewing materials over multiple purchases

Savings is most typically plural in the sense that you are using it:

Our savings on needles are significant

These are my coupon savings

I have $300 in savings

What you want to do is replace "coupon" in the second example with "Material Costs":

Material Costs Savings — the savings on "material costs"

The use of "costs savings" is probably what makes this seem strange. The idea that you are accumulating savings through costs is counter-intuitive. But you aren't saving through costs, you are saving on costs. The alternatives are not quite what you are looking for:

Material Cost Savings — the savings on the price of "material"

Materials Cost Savings — the savings on the price of "materials"

These don't make much sense to me. I don't think "savings" works well with "cost".

Materials Costs Savings — the savings on "materials costs"

This works but the extra plural is even more distracting. This form would work better with other objects, however:

Needle Costs Savings

Car Costs Savings

You can to try to use a hyphen to reduce the awkwardness:

Material-Costs Savings

But I would just stick with Material Costs Savings.