Honshu, map

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 hot and humid currents of tropical origin.
The town is located on the coast, in the Seto Inland Sea, an area relatively protected both from the summer monsoon and from typhoons.
In 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 which is typical of big cities. 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 because of the thermal inertia of the sea and because of the lower frequency of periods of bad weather.
Here are the average temperatures.
Osaka - Average temperatures
Min (°C)33611162024252216105
Max (°C)101014202428323329231812
Min (°F)373743526168757772615041
Max (°F)505057687582909184736454

Rainfall is quite abundant, in fact, it amounts to 1,300 millimeters (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 rain frequency: on average, among the summer months, it is the hottest as well as the driest and sunniest. Here is the average precipitation.
Osaka - Average precipitation

From June to October (but especially from August to early October), Osaka may be affected by typhoons, the tropical cyclones of Southeast Asia, though the 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 3rd.
On the whole, the amount of sunshine is decent in winter, but it never becomes very good on average, however, there are two relative maxima, one in spring (April-May) and the other in August, after the Baiu rains, with 6/7 hours of sunshine per day. Here are the average sunshine hours per day.
Osaka - Sunshine

Osaka Castle

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