The peak seasons that sell out early
Cherry blossom season in spring, Japan's Golden Week holiday stretch in late April to early May, and the Obon holiday period in mid-August all see a surge in both domestic and international travel, and central Osaka's best-located and best-value rooms in these windows can sell out months ahead rather than weeks.
Outside peak season
Outside those specific windows, Osaka's large hotel supply means booking a few weeks ahead is usually comfortable, though popular capsule hotels and highly-rated smaller properties can still sell out earlier than a standard business hotel.
Flexible vs non-refundable rates
Non-refundable rates are typically cheaper but remove any flexibility if plans change; a fully flexible rate costs more but is worth it for the part of a trip with the least certainty, particularly early or late in an itinerary where flight delays or plan changes are more likely to matter.