I had my hair transplant 2 days ago. I have a tattoo booked in for tomorrow. Is this OK?

Thank you for the question.
As long as the tattoo is not going to be in the same area as where the hair was relocated you should be fine.
You need to make sure that nothing comes in contact with the new hair that has been transplanted. 
I educate my patients not to take any risks and that by day 10 typically things are more secure from damage.

I advise that you consult your surgeon before getting your tattoo but generally, as long as the tattoo is not in the same region as your transplant, it should be okay to go ahead with the tattooing. 

It is best to ask your surgeon about post operative care as each doctor has their unique regimen.  

As long as your tattoo is not near or in close proximity to your hair transplant (where they have taken hairs out and put hairs back in) I do not see any problem in getting one. 

Are you talking about a tattoo to your scalp?  If so, that is too early as you should wait until everything is healed. If you are talking body tattoo, that's ok