I can't seem to find what the unique identifiers for the Mekanism ores are. I assumed that they are Mekanism:OreBlock.<OreName>, for example, for Osmium Ore, it would be Mekanism:OreBlock.OsmiumOre. But this does not exist in the item registry (yes, my mod is loading after Mekanism). What are the names for the ore blocks so that I can reference them?

And this question also follows for dusts, ingots, clumps, etc... I can't figure out these multi-id items. I will edit the title for clarification.

Using mod tweaker is useful. Type /mt hand and it should give you the name and meta data you need.

@bookerthegeek Thanks for the help. I got this result from that command while holding a block of Osmium Ore:

What does this mean by "is in"?

Mekanism:OreBlock is the id of the block itself.
ore:oreOsmium is the ore dictionary entry that the block is in.
You would probably want to use the ore dictionary entry in case another mod also adds osmium ore.

Yep. The "is in" can have multitude of entries as items can have multiple ore dictionary entries