does anyone know if you can rotate the onstar antenna or if it can just be rotated so the mast faces front instead of back.

 

I'll ask. Why would you want to do this ?

If you can, I'll bet it would defeat it's aerodynamics design. In other words, you may succeed in turning it around but may also create some unwanted wind noise.
Best to leave it as it's designed to be mounted.

 

I have a spoiler for the back lift gate but it won't fit as it hits the antenna mast.

 

I don't have one, but I thought the mast could be pushed down, (not in) or removed.

 

Plenty of HHR's around with a stock style spoiler and Onstar mast. They don't seem to have any problem with the spoiler hitting.
Recheck your spoiler mounting place. Sounds like it's incorrect.

With my hatch open, I've got about 3 full inches clearance between the SS spoiler and the mast.
Here's mine with the hatch closed.

 

I have the hoop spoiler which sticks up farther, maybe I will need to sell the one I bought and buy this type.

 

Is yours the SS spoiler type?

 

Do you want to have OnStar? The parts books show the mast separate from the base.

 

I would like to keep Onstar.

 

Like I stated above, there are thousands of HHR's out there with spoilers. Both the hoop style and the SS kind like mine.
If your's is hitting, you obviously have something mounted incorrectly. Either the spoiler or the mast. If your's has the factory Onstar mast, then my bet is you have the spoiler mounted wrong.
Maybe posting a picture would help us help you out.