Generally agree on the above, altough the cutoff for delivering bags and checking in for a flight between OSL and HEL is 30 minutes, which technically gives you a few minutes extra. I personally NEVER would fly on two separate tickets with just 75 minutes in between, but if all goes your way, it can be done, technically and hypothetically speaking. Nevertheless, I would seriously advise you to take one flight earlier with either SAS or Bjørn. There are usually plenty of flights during the day, so it would probably only take up to one hour extra. One hour you're going to need on OSL during summer.

You can check in online with Bjørn, the link is already there. OLCI does not give you the option of seat assignment though, so you may end up apart from each other. You can change this again at the check in machines in OSL, if you have a bit of time left.

Talking about the luggage, my experience is that it takes, on average, 30 minutes to get the luggage. OSL is very, very slow, both SAS and the others, at handling luggage. My all time low is actually 1,5 hours before I had my priority luggage tagged bag.

My experience is also that Bjørn is quite strict with deadlines for delivering luggage. You may have to print out your luggage tag at the check in machine anyway, put it on yourself and deliver it at a self service kiosk. Seems like the self service kiosk is not to be avoided when flying Norwegian.

Fortunately, there are plenty of those. Just go to the nearest elevator or escalator up from arrivals. You should actually be very close when walking out of domestic arrivals. Just look left or right, whatever you prefer. Kiosks are located on one side of the terminal.

Keep in mind that waiting times during summer can be horrible due to hordes of Norwegians flying to vacation destinations. And most Norwegians pretend to never have flown before and pretend that if they're not in a hurry for their charter flight, you aren't in a hurry either. Not sure if fast track is a good idea, as it's flooded with all the SK silver people.