We used Orkney Aspects. Just be aware that any of the companies are likely booked up already. We ended up on a wait list, because there are only so many vehicles on the island and they needed to try and secure an additional vehicle. They did, and we got an email with a 24 hour window to book -so make sure you check your email account. They were very organized and very good with communication.
DCL had some nice looking tours as well - you can probably find some trip reports that include DCL Kirkwall excursions.
Kirkwall was one of the favorite ports for us! Whatever you do, make sure you have a good jacket and hat. It is cool/cold but it is the wind that will be the worst! We had lightly insulated wind-proof jackets with hoods - best overall purchase for the trip (Iceland, Stavanger Norway, and Kirkwall were all very windy and quite cool/cold).