The reason it doesn't work in the OC, is that it was never installed there. Unlike the Override forlder, the Erf folder is more for easy storage than anything else. The Hak folder is where the game searches for a Hak whenever it is asked to look for one.
To install the Greatsword and Katana, you would need to open the OC modules in the toolset, go to the Edit -> Module Properties -> Custom Content tab, select the right hak from the drop-down menu and save the module (under a different name).
You would then have a module which allows you to use these Katana and Greatsword models.
However, they wouldn't be available anywhere yet. I'm assuming the .erf file has some sample specimens, however. Erf files are meant to be imported into a module. So you'd go to File -> Import and select the right erf file. Overwrite things if you have to. You would then have a number of these weapons available on the pallete.
However, having them on the pallette doesn't mean your character can get at them. So you'd have to open an area you know the character will soon visit (View Area), open the items list (Paint Items, to the right), and since the items you want to add are non-standard, select Custom. Then they'll most likely be found under Weapons -> Bladed -> Greatsword and Weapons -> Bladed -> Other.
Click one of those and drop it on the ground (or give to a merchant, more complicated though), then save the module (under that different name).
After those steps, you should be able to use the new weapons in one single chapter of the OC. Once your character has the weapon, though, you'll no longer need to do anything other than add the Hak to the Custom Content list for the other chapters.
Then again, if I remember right, a chapter is only unlocked for editing after you finish it once already...
So what it comes down to is it's likely not worth it for the OC, though you can consider doing this for other standalone modules, and can definately add these kinds of sword to modules you make yourself.
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