Chocolate won’t harden up on my cookies

The problem is the “small amount of oil”. If you measured wrong or just eyeballed it, you now have a chocolate spread instead of a solid coating.

Storing the cookies in the fridge may or may not be enough to solidify the chocolate coating, but apart from that, there’s nothing you can do to change the chocolate. Depending on what matches the cookie flavor, dipping the sticky part in something like grated coconut or ground almonds may at least protect your fingers somewhat.

It may have something to do with the temperature that the coated cookies are stored at, I mean, some room temperatures can be significantly higher than others. Perhaps the chocolate will harden in the fridge?

If the temperature is not the case, get this: You can drizzle or brush on another, likely thin, layer of pure melted chocolate (who doesn't love more chocolate :p) over the existing chocolate coating.

If my calculations are correct, pure melted chocolate will harden on soft surfaces (like Nutella) just as well on hard surfaces (like more solid chocolate).

Sounds like you might have messed up the tempering of the chocolate.

In order to get chocolate that doesn't melt easily at room temperature, you need to properly temper the chocolate. This is a very complicated process involving holding the chocolate at specific temperatures for a specific period of time. Additionally, adding oil to the chocolate might have resulted in the tempering time and temperatures being altered as a result of the adulteration of the chocolate.

You may need to throw away all of the chocolate and making new, unadulterated chocolate that has been properly tempered, but if the problem is that the chocolate hasn't been tempered properly and the adulteration haven't altered its characteristics too much, you might be able to save it by scraping it all off of your cookies, then melting down some chocolate and adding your adulterated chocolate into it before following a proper tempering process.

In addition to Stephie's answer, when melting and re-cooling chocolate, it needs to be tempered properly. That refers to how the chocolate is cooled and "seeded". The fats in chocolate can have several different crystal structures. That affects the texture of the chocolate. The particular structure your chocolate has depends on how it's tempered. Here's one method from Ghirardelli:

Grate or chop the desired amount of chocolate. Place two-thirds of the chocolate in the top pan of a double boiler. Heat over hot but not boiling water, stirring constantly, until chocolate reaches 110°–115°F. Place the top pan of the double boiler on a towel. Cool chocolate to 95°–100°F. Add the remaining chocolate to the top pan, stirring until melted. The chocolate is now ready to be used for molding candies, coating, or dipping.

Adding the unmelted chocolate is adding a "seed" of the desired crystal structure so the melted fats will match the structure of the unmelted crystals.

I would say take one cookie, turn it upside down and place it on another cookie. Unless the mixture is too fluid you should be able to eat the cookies this way.

Maybe try sugar powder or cocoanut flakes to bind the oil? If that fails you can still take one cookie, turn it upside down ....