Cambodia's rainfall will start to decrease, especially towards the end of the month with the dry season just around the corner. Across the country the flora and fauna is in full bloom and the moats at the Angkor Temples are full of life (avg temp: 27°C). October is also one of those months that frequently defies the averages with the outcome of the weather, especially in the 2nd half of the month, very much depending on the severity of the summer monsoon.
One great benefit of travelling in Cambodia at this time is that the Angkor Temples will be far less crowded.
On the south coast, in Sihanoukville and Kep, rain is still a possibility although this will become less frequent as you head towards November and the start of the dry season.