Reservation for Shinkansen w/ JR Pass in Dec

Hello, I just booked plane tickets for Osaka with the trip lasting from Dec. 23 - Jan. 1. I'm travelling with a group of 6. We're planning to spend the first 5 days in Tokyo, and the next 5 in Osaka. We would buy 7-day Japan-wide JR passes. So on December 23, we'll be taking the Shinkansen from Osaka to Tokyo. On the 28th, we'll go back to Osaka. I understand that the New Year Holidays may be a difficult period for commute especially going out of Tokyo. Would it be possible for me to reserve seats for Dec 28 immediately in the airport on Dec23 once I buy my JR pass? Also, would it be extremely difficult or even impossible to get a seat in the Shinkansen on the 28th given the holidays and that it's a Saturday? Any help would be greatly appreciated. Thank you!

Yes, you can reserve seats for Shinkansen as soon as you have exchanged your JR Pass (ask the staff whom you exchange your pass with so no need to line up twice). It is not a guarantee, but I think the chance you can get 6 seat reservations is quite there if you book the earliest Hikari train of the day. Usually Japanese people will prioritize Nozomi trains since they are faster. I have done it multiple times including during the New Years period with a group of 5 people.

There was no need to buy the JR Pass, if you just go to Tokyo and return !!!
Yes, it is highly recommended to make the reservation for the period you want to travel.
From a short time you can make the online booking, but I believe that Osaka / Tokyo / Osaka is not included, sorry for not giving a certain information.

Thank you for the responses, I'll take note of that :) Can't wait to go back to Japan!! Thank you again!

With the rail pass (7-day national version) you might make a small saving, but you cannot reserve the seats until you get to Japan and get the actual pass.

Saturday 28th may not have any seats for reservation (it is a Saturday) and only non-reserved seats by the time you arrive (3 hours standing is not so pleasant). For 2018 shinkansen bookings for the Saturday and Sunday before New Year were 100% completely sold out. Better is to book before you arrive - the SmartEx website allows advance bookings one month prior.

I use the service, and as a user I am allowed to book a week earlier - a small benefit to get ahead of everyone else buying their tickets at the ticket office.