Map from Google - Osaka

The climate of Osaka, a Japanese city in the southern part of Honshu Island, is temperate humid, with quite mild winters and hot, moist, and rainy summers. Like the rest of Japan, the city is affected by the monsoon circulation: in winter the northwest cold currents will prevail, while in summer they will be replaced by the hot and humid currents of tropical origin.
The town is located on the coast, in the Seto Inland Sea, an area relatively protected from summer monsoon and typhoons.
In the winter there are fairly mild periods, with highs above 10 °C (50 °F), alternating with cold periods, with wind and rain, highs around 5/7 °C (41/45 °F) or less, and possible falls of sleet or snow. Snowfalls, however, are usually light, as are nocturnal frosts.
Summers in Osaka are particularly hot and humid, also for the heat island effect of the big city; there can be periods of good weather, with maximum temperatures around 35 °C (95 °F) or above, tropical nights and high humidity, but also periods of bad weather, due to the summer monsoon. August is the hottest month, both for the thermal inertia of the sea, and for the lower frequency of periods of bad weather.
Here are the average temperatures.
Average temperatures - Osaka
OsakaJanFebMarAprMayJunJulAugSepOctNovDec
OsakaJFMAMJJASOND
Min (°C)33611162024252216105
Max (°C)101014202428323329231812
Min (°F)373743526168757772615041
Max (°F)505057687582909184736454

The rains are quite abundant, as they amount to 1,300 millimetres (51 inches) per year; winter is undoubtedly the driest season, at least in quantity; the rainiest months are June and July, ie the first period of the rainy season (called Baiu). In August there is a certain decrease in the frequency of the rains: on average, among the summer months, it is the hottest, as well as the driest and sunniest.
Here is the average precipitation.
Average precipitation - Osaka
OsakaJanFebMarAprMayJunJulAugSepOctNovDecYear
OsakaJFMAMJJASONDY
Prec. (mm)45601051051451851559016011070451280
Prec. (in)1.82.44.14.15.77.36.13.56.34.32.81.850.4
Days771110111211810976110

In the latter part of summer and in early autumn, from August to October, Osaka may be affected by typhoons, the tropical cyclones of Southeast Asia, though this area of the Seto Inland Sea is relatively sheltered (however, in September 1934, the city was hit by a typhoon that caused a lot of damage).
The cherry blossoming in Osaka occurs around April the 3rd.
The amount of sunshine in Osaka, all things considered decent in winter, never becomes very good, however, you can see two peaks, one in spring (April-May), and the other in August, after the Baiu rains, with 6/7 hours of sunshine per day.

Osaka Castle

The sea in Osaka is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, and it gets pleasantly warm in August.
Sea temperature - Osaka
OsakaJanFebMarAprMayJunJulAugSepOctNovDec
OsakaJFMAMJJASOND
Sea (°C)171516171922252726232119
Sea (°F)635961636672778179737066