I am planning to go to Oulu in February for 4 days. What are the chances of seeing Aurora Borealis and are there some tours available? Would there be a significantly better chance of seeing the aurorae further north in Rovaniemi?
Your odds are good but not great.
The season is good, since while the peak season for auroras in Oulu ("O" in the graph below) is April, but there's a fair amount of activity in February as well.
However, the incidence of auroras drops rapidly as you go south. Per this page from the Finnish Meteorological Institute (in Finnish), in northernmost Finland, 3 out of 4 nights have auroras, but around Rovaniemi that falls to 50% and in Oulu only 1 out of 3. Remember that these are statistical averages: if it's cloudy, you won't see anything, since aurora are far above the clouds.
And yes, there are tours from Oulu, here's one selected at random. Rovaniemi is a much larger tourist destination though and will have more options.