Yes, each bass has a nut that fits. Maybe you could find a MM forum and ask them where to buy nuts? Does Ernie Ball have a web site with forums? Maybe someone they could help you. Maybe Ernie Ball even sells the nuts on the web site? You'll never know unless you try.
Or, another thing to do would be to find the old nut, take it to a guitar shop and say "Sell me a nut like this for my OLP MM2." Or take the bass with you, and say, "I need a nut to fit this bass. My dog ate the old one."

They will probably have a cheap plastic nut that fits.
Or if the nut you took out of the other bass almost fits, you could buy one just like it, only taller, and sand it down until it fits and is the right height.
Or you could make a new nut out of a piece of bone, or other material. You can buy material for making nuts--either bone or manmade materials-- from Stewart-MacDonald (stewmac.com). They even sell finished nuts in common sizes (Fender, Gibson, and Martin). Yours might be the same as one of those.
Here's a link about how to make a new nut--useful even if you just need to sand one down:
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Have fun,
Ed