It's been 12 months since my hair transplant. I still have empty spots and the hair is still thin! Is it OK or not? (Photo)

If you are not satisfied with your surgery results, it is best to follow up with your doctor and express your concerns.

It is difficult to tell from your photos exactly what is going on but it does look like a nice result.Keep in mind that there is a limit to how dense an area can actually become through transplants,and the hairs will continue to thicken up through the growth cycle.I would recommend following up with your surgeon and expressing your concerns.

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During a single hair transplant a
maximum density of 40% is the usual goal. In order to increase the density a
second hair transplant would need to be done with grafts being placed in
between the existing transplanted hairs. 
This should give you a solid hairline and front if that is the “master
plan” intended by your surgeon. You need to discuss this with him and he can
review it based on the number of grafts and density he has planned to give you.

 I think it seems to be low density and un successfull result for the 12 month post op.. you may need a revision surgery to get more density .The most outstanding reason for poor growth and ineffectual  hair restoration results is insufficient number of viable graft placement as a result of intra operative  damage of follicular units ( during insertion or high transection rates during harvesting, long time of grafts out of body, dehydratation, poor FU storage conditions like high room temperature and storage fluids etc.)

At 12 months past your transplant it's unlikely you will see any further changes to your hair. Since you're still unhappy you should talk to your doctor, it is possible that there was a miscommunication in what your results would look like. In general transplanted areas will not be the same density as normal hair, and it's hard to say whether your transplant was a success without seeing before pictures. 
Many patients get a second transplant simply to fill out and add further density to the recipient area, this may be your best option to get results you are satisfied with.

You should begin to see hair growth at around 3-6 months. If 1 year has passed and you are not happy with the density, I recommend that you follow up with your hair tranplant surgeon or a hair transplant specialist for an in-person examination of your scalp. 

At one year you should have most of your growth.  This would be a good time to visit your surgeon, review your preoperative photos and discuss your concerns.

If you have empty spots after one year, go back and meet with your surgeon and discuss your expectations with him and what you grew out