I'm sorry, but I found the wording of your post a bit confusing. I am taking it to mean that you are as yet unsure of your exact arrival time. If this is the case, then I would hesitate to make any early ADR for your arrival date. You will usually reach your WDW resort within 90 minutes of arriving at the gate in Orlando. Then you can assume about 30 min. to get checked in and receive your KTTW cards.
Then, depending on which resort you are at, and if you are taking WDW transportation, it could take anywhere from 30 min to 90 min to get from your resort to the Poly.(depending on how long you had to wait for the bus for the MK, and how long you had to wait for the monorail or resort launch).
Then you have to factor in the time of your ADR. I believe the earliest ADR for dinner is 5:00pm. I'm assuming you want an early ADR in order to get any useful time in the MK that night. Realize that it will take you at least a couple of hours to be seated, eat, then get back to the MK.
You really need to give yourself a cushion of time to allow for possible delays with your flight and other transportation during the day. Long story short (too late!!) I would recommend that you not have any ADR that requires a credit card to hold it on your arrival day, and no ADR within 6 hours of arrival in case of unforseen delays.
If Ohana is really important, then make it for another day. Perhaps you could make a later ADR for a TS restaurant inside MK for your arrival day. Unless you are arriving at a very quiet time of year I would not try to do a walk up at a MK restaurant or at Ohana. We usually opt to just do a CS meal on our first day, unless we are arriving in the morning.