When both GT6U and Mek are installed, there are two type of Osmium ore. GT6U didn't unify the Osmium ore and it's Material.
one is gregtech:gt.meta.ore.normal.default:760 the other is gregtech:gt.meta.ore.normal.default:8719.
The diction of the former is -oreOsmiumElemental
-oreVanillastoneOsmiumElemental
The latter's diction is -oreVanillastoneOsmium
-oreOsmium
The former is the true Osmium. You can use it to make tools of level 4 and anything which need it. The other can make the tool of level 2, and you can't use it to make things about GT6U. Obviously, they are different things, but can be generated in a world equally. I have tried to delete the latter by modify configuration file. However, i couldn't find the item of OreOsmium from worldgenerationnew.cfg. It really confused me.