Like mentioned above, I'm from Oulu, but I spent my childhood a bit to the north closer to the Swedish border. Due to this I have some Lapland influence in my dialect when it comes to word choices. However my parents are both originally from Oulu, so that's what I heard most when growing up. From pronunciation perspective I think I could fool anyone that I'm paljasjalkainen ("bare footed" i.e. native born) Oulu dweller.
The most distinguishable trait in Oulu dialect is double consonants. We often stretch single consonants and change consonant pairs to a double consonant. The classic example goes: "Oo
kko nää Oulusta? Pelekää
kkö nää po
lliisia?" which in standard is "Ole
tko sinä Oulusta? Pe
lkäätkö sinä po
liisia? (Are you from Oulu? Are you afraid of the police?). As you can see, the you personal pronoun is written as "nää" although I personally prefer "sää". Similarly minä is "mää". In contrast the forms used in many southern dialects are "mä" and "sä", showing yet another stretching of the words. This time strething of vowels instead of consonants.
First trait of the Lapland dialects others notice is adding H to a lot of words. This is called "puhua H:n päälle", "to speak on the H". The place of the H in words varies from subdialect to another, but you can still hear when the H is definitely in the wrong place. Southeners sometimes try to imitate it with hilarious results. They seem to miss the spot almost always.
Here's a champion both showing how it's done and lameting on the bleak future of the habit:
(yes, they actually had a competition)
I can fake some Helsinki dialect, stadi slang (from Swedish stad = city), but I'd be easily caught by someone actually living there. Thus I should leave the pecularities of this to someone who can actually speak the dialect, but here's some notes on its history. While most of the vocabulary comes from Finnish, a lot of the words have origins in Swedish and English. This hasn't always been the case. When Finland was a Grand Duchy of Russia the slang had mostly Swedish words mixed with some Russian and Finnish as half of the city's population was Swedish speakers and the government officials spoke Russian. After independence both of these languages' influences started to drop while being replaced by Finnish words as the demographics of the region switched from 50-50 split to a majority of Finnish speakers. Later at the end of last century English started to mix in and now the language is mostly distorted Finnish with English and Swedish words mixed in.
Edit: while maybe not the best formatted page on the subject I'd like to recommend
https://en.wikipedia.org/wiki/Colloquial_Finnish for those more advanced learners who are at a stage where they might listen or speak colloquial Finnish in addition to the standard one. The entry should give you an idea on what kind of reductions can be and are done in pretty much every dialect.
