If coming from Spain you might find that most things are cheaper back home (with the possible exception of exclusive wines).
You can check prices and assortement here: http://www.tax-free.no/5106/produkter/ (only in Norwegian, I'm guessing they know their prices are too high for foreigners).
Duty free alcohol prices in Oslo are competetive with other airport duty free shops. Always on the lookout for better deals, but rarely worth the hassle of carrying on the plane. An exception might be beer. But whether duty free prices are better than you can get in shops in Spain is another question. Just remember that duty free bottles are 1litre, while shops are 0.75 litres and you have to pack safely in checked in baggage - you won't be popular if a bottle breaks and damages other peoples luggage. Certainly not advised by airlines.
Wine prices at the Oslo duty free shop are generally not that competitive. There are some better deals to be had, but you really have to do the research. A few days ago, we were considering a particular bottle and checked the Vinmonopolet website only to find out that the full price with tax was only 8 NOK more than what the duty free shop was charging. Definitely not worth hauling the bottle into town for 8 NOK!
Most wines offer a savings that's close to the 45 NOK per-bottle tax, and a few offer even better deals. But there are definitely cases where the markup by the duty free shop eliminates the tax savings entirely.
I live in Oslo and I find that generally the prices in Oslo are more expensive than the duty frees of other airports. I usually buy at the departure duty free shop. Actually, most prices at Oslo airport are highly inflated and the service and quality leaves a lot to be desired, especially when it comes to food.