My dear Rania, I live about 100 km south of Oslo and I have seen much of Norway and Sweden by sailboat, camping van and light aircraft. The weather in Norway and Sweden is surprisingly pretty much the same and ... extremely different, but not across the border but rather, the actual location.
Both countries have a different climate, from the coast (maritime) to inland (continental). Perhaps what is most noticeable is the fact that the Gulf Stream brings mild water and keeps the Norwegian fjords ice-free, all the way to the North Cape and the Russian harbour of Murmansk.
The general weather pattern is pretty much the same as in UK: low pressures move in from the Atlantic along the polar front, creating a mild but moist weather most of the time. But while UK is relatively flat, Norway has those big mountains and as the rain meets the south-west coast, it rises, causing pour down on that coast and this is why the town of Bergen, on the Norwegian west coast is the most rainy place in Europe.
But then, on the Oslo east coast, on the lee side of the mountains, the climate is much dryer. In fact, the most sunny place in Sweden is the island of Koster, nearby the Norwegian border. Simply because it also enjoys the pretecting lee side of the mountains and, as an island (with a weather station!) is is off the summer sea breeze bringing clouds over the mainland.
During the day, the sun heats the earth. At night, that heat radiates back into space. The balance of which is called, the sun's net radiation. It is always positive (more gain than loss) at the equator and always negative at the poles. In-between, it is positive during the summer.
So, a high pressure over Norway and Sweden during the summer means, a warm temperature and, in the winter, a very cold one. But there is more to it! Once you are north of the polar circle in Sweden or Norway, the sun doesn't set near midsummer. It is called, the midnight sun! But when there is a high pressure with clear sky and little or no wind, the temperature increases and increases since there is no heat loss at night! And that is why in the north of Norway or Sweden, especially inland, the temperature can reach as much as 35 C during a summer high pressure!
Mind you, if the next day, a low pressure moves in, the temperature may drop as low as 5 C and you might even get sleet precipitations ... in the summer!
The bottom line is: the weather in Sweden and Norway is pretty much the same as general. But from region to region, it can vary very much!