How can I make my Oreos crispy again?

Since the problem is caused by the humidity absorbed by the cookies left opened, the solution would be to remove that water. Putting in an hoven at very low temperature could help the process but there's a risk of melting the inside. Another option would be to use a dehumidifier. The problem with this solution is that usually this kind of appliance are supposed to suck humidity inside a room and thus it won't target specifically your cookies unless you manage to find a way to contain the target area. Maybe you could try with some small chemical dehumidifiers and seal them with the cookies in a bag for a while. There are some options but ultimately if you think about it, are they really worth the effort just to clean your conscience :) ? In my opinion you should just mark this lesson and use the cookies for another preparation like a milkshake as other suggested or a cake, afterall they're still absolutely edible.

Try putting them in a bag of rice. It's a widely known trick when dealing with wet electronics, rice absorbs moisture and should make your cookies crispy again.

There might be some grains of rice stuck to the filing afterwards, though.

Preheat the oven to 300F. Scrape out the filling into a bowl. Put the cookies on an ungreased, filling side up in case there is still a bit of filling on the cookie. Bake for 5 minutes. The cookies will come out soft but that's okay. Let cookies cool completely. As they cool, they should harden. To reapply filling, heat filling in microwave in 5second intervals until filling is soft and pliable. (Stir filling each time, much as you would when melting chocolate.) Scoop filling into icing bag and pipe onto cookies. The filling solidifies quickly so work fast. If you melted the filling too much, wait a while before application.

Just pop them in the freezer for 30 minutes (overnight is better) and eat them cold.