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. See the seasonal guide for exact timing on each of these.
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 — particularly around weekends even outside the major holiday periods.
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.
Where to actually book
Once you know your dates, cross-checking a property across a couple of different booking platforms before paying is a genuinely useful five-minute habit — the same room can show meaningfully different prices depending on where you look.