Press down on it and turn. If that doesn't work try using a grip as if your trying to pry it as you try to turn it. This has worked when my oil cap just spun freely. If those both fail you can use a set of channel lock pliers and squeeze it hard, it will come loose but the cap will be trashed.
My oil cap top separated and I could turn the inside to loosen. Let us know how you make out on this.