it should be, as long as the stopper doesn't have a huge hole in it from a large gauge needle. For instance, if you had stuck it a year ago with a 18 gauge needle. I'm sure you probably used a 23 gauge or similar. The stopper should still be air tight with just 1 little hole.it should be sterile if you used proper technique when you punctured the stopper.
One thing you can do is turn the vial upside down. Look for bubbles coming up through the oil solution. If you see bubbles then that means that the stopper is not air tight. In that case I would toss it.