Spring and autumn
Light layers cover most of spring and autumn — a t-shirt with a jacket or cardigan for cooler mornings and evenings is usually enough, though early spring can still turn sharply cold. An umbrella is worth packing in either season, since both carry a reasonable chance of rain.
Summer: plan for humidity, not just heat
Osaka's summer is genuinely humid as well as hot, and breathable natural fabrics do noticeably more work than technical synthetic layers built for dry heat. A portable fan or cooling towel is a common local habit worth copying, and reapplying sunscreen matters more than the temperature alone suggests.
Winter: cold, not extreme, and usually dry
Winter calls for a proper coat rather than heavy layering underneath, since indoor spaces are well heated and you'll be moving between warm interiors and a cold but rarely freezing outdoors. Snow is uncommon in central Osaka, so heavy winter boots are rarely necessary.
Year-round basics
Comfortable walking shoes matter more than anything season-specific — Osaka is a walking-and-train city, and most days involve more steps than people expect. Slip-on shoes are genuinely useful given how often shoes come off indoors, and a compact daypack keeps your hands free for the pace day of temples, markets and train transfers.