I am wondering if it is better to change money (euros to nok) in Paris in an exchange shop or in Oslo. I heard that Forex has got a good rate. I checked in the place in Paris and one euro is about 8,40 nok. I do not know if I will get better change in Paris or directly in Oslo, in Forex for instance

Are we talking cash-to-cash exchange?? ( don't do that, far to costly. Buy with debit or credit cards or get some cash at an ATM).
The cash rate at Forex is here: http://www.forex.no/ Right now you sell € for 8.15 nok and buy € for 8.99 NOK. I guess the 8.40 rate you saw is the official rate used for electronic money only. You can't exchange cash for that rate. But rate for plastic cards will be within 1-2% from that rate unless you come from a country where the banks rob their customers.
Ps if you make an exchange between a "small" currency and a "huge" currency it is normally - obviously - cheapest to do it in the homeland of the "small" currency.

Many thanks Bjoern for your help and explanation
So, 8,99 is the rate they give selling euros to you!!! But 8,15 is the rate they give you if you sell your euros to NOK. So I guess it is better to change in Paris that they give 8,40 por one euro. If you know any exchange place with better rate in Oslo please tell me

If they really give you 8.40 without you having to pay a commission then do it in Paris, but I am as as I explained that what you have seen is the official average exchange rate - the one you see in newspapers on the finance pages. I am pretty sure that you will also get only a bit more than 8.
Forex bank is one of the cheapest exchange offices in Scandinavia. Difference between buying and selling rates on Scandinaviar currencies are high in these days as some are trying to press the DKK to raise its value. If that happens the NOK and SEK will also rise.

Many thanks Bjoern. So, I will try to do the exchange in Paris, according what you explain. What I was wondering that in Oslo, like in many cities, there is a street with small shops, who offer you better rates. When you were talking about 'small' currencies and 'big' currencies. Where you would put NOK?

I must admit that I do not know present Oslo well enough to have noted if the have those main street exchange shops. I know that here in Copenhagen they exist and are COSTLY - do look like that but the rate the advertise is the opposite of the one toursist need (like the 8.99 and not the 8.15) - they also write no commision - but that is (written in tiny letter) for SELLiNG the local currencey only. Make sure you really have understood the deal before you use these places.
In DK Forex bank (that also is on the main street and central railway station) is the exception - their deals are better than any/most unless if you are changing large amounts (1000s of € where real banks are/may be cheapest) - but still - use plastic and save min 5%

Really no need to get lots of cash, since A. cards are accepted everywhere and B. pickpockets are an increasing problem in touristy places. Credit cards are accepted everywhere, you might find places that don't accept foreign debit cards, but it's not a major problem.