Cat Bath

This morning, I let my cat out and he was rolling in the dirt and having a really good time. Now that he has come in, his fur is rather oily and not soft from rolling in dirt. Unfortunately, I have no grass and my landlord doesn't want to grow grass. Anyway, is he going to lick it off, or should I give him a bath? I don't have cat shampoo, so is it ok to use regular Dawn soap?

Since it is oily I would give him a bath or prevent him from licking something nasty. Dawn soap is fine, it is a safe and gentle soap. Make sure you don't get any in the cats ears or eyes (I typically don't go past the neck unless there is a need). I have found putting it on a washcloth and sudding up before rubbing the cat helps prevent globs of soap in spots. Rinse well so there isn't any soap residue left on your cat. 

Make sure your cat is fully dry before letting back outside. I usually wrap in a towel then change towels two or three times to get most of the water off. If your home is warm then you can let the cat air dry once they aren't dripping. I stay away from hair dryers because my cats are scared but I'll set up a space heater and trap them in the room with it until they dry off. 

Ok, I was going to let him back outside again, so I am going to wait and not have to give him two baths in one day. I'm sure that's not good for them.

You should give them a few days between baths for their skin to regain it's natural oils. Otherwise they can get dry itchy skin. Sometimes you need multiple baths in a short span for specific reasons but that should be a once in a while thing.