When flying from Johannesburg to Oulu, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Johannesburg is roughly greater than that of Oulu. You can assume this to be the case over the next 19 days.
We forecast the temperature to be around the 26 °Cs in January, on average. The temperature in Oulu is generally colder than in Johannesburg. Book flights in a timely manner in correlation to your departure date.