For the Shinkansen (Bullet Train) from Tokyo to Osaka in 2026, you do not necessarily need to book in advance, but it is highly recommended during specific times. For most of the year, the "Nozomi" trains run so frequently (every 10–15 minutes) that you can simply walk up to a ticket machine at Tokyo or Shin-Osaka station and buy a seat for the next departing train. However, you MUST book in advance if traveling during "Peak Periods" like Golden Week (early May), Obon (mid-August), or New Year’s (late Dec–early Jan). During these holidays, the unreserved cars can be standing-room only. For 2026 travelers, using the Smart EX app allows you to book and change seats digitally up to 30 days in advance. If you have a JR Pass, you can also book your reserved seats for free at any JR ticket office once you arrive in Japan, ensuring you have a guaranteed spot and space for any "oversized luggage" you may be carrying.