Rarely do (cannabis) visitors to the Netherlands (apart from Northwestern Germany) make the trip to Groningen, which may be a bit of a shame. With something like 35-50% of its population being students at any given time, it's really a youthful and vibrant city with a much more laid-back, welcoming feel, as opposed to Amsterdam's relative hostility in some cases. We've got 13 or 14 coffeeshops (not sure) which almost all allow smoking tobacco (even cigarettes in plain sight) and many bars are very open towards smoking weed inside.
Nevertheless, I have seen a few posts (and comments on the guide) listing Oasis as one of the highlights of their trips to the Netherlands, singling out this establishment as one of the best across the country.
I couldn't agree more; it's pretty small, but most Groningen coffeeshops are - and rarely is it so full you won't find a seat. As opposed to most of Groningen's coffeeshops, which have no windows or very obscure windows to the street, Oasis resembles a classy cafe (much more akin to, for instance, Amnesia, De Dampkring, Siberie etc. from Amsterdam). Many Groningen coffeeshops have a kind of den-like, dive feel to them where your smoking is kept in secret. Why shouldn't you be staring at the sunshine through a glass window, enjoying your spliff and not giving a fuck if the grannies can see you through the window? In Holland, I don't want to have to hide my weed-smoking.
Either way, I digress: Oasis. They have about 20-25 seats I guess, with a few tables scattered around and a bar. Chillums, bongs and grinders are available to borrow free of any kind of deposit; there is also wireless internet. The decor is very chilled and cafe-like with occasionally changing paintings and graffiti adorning the walls.
The smoke is sold pre-bagged in bags of 5e or 12,5e (except some specials - there will be expensive weed and hash specials sold by the gram, that get slightly cheaper as you buy more - 1 gram extra usually drops off 0.5-1€ from the final price, so really you're only getting 3 grams for 10€/g instead of 1g for 11€/g). They typically stock about 6-8 different types of weed and 4-7 kinds of hash, all usually top quality. The hashes follow the usual price range of 5€ Maroc to 11€ Nepalese. Weed goes for 8.70-12€/gram (they have really funny decimal prices, like 9.10€/g - why not just 9€?).
By Groninger standards, this is reasonably expensive, but the upper half of the Oasis menu will usually blow everything every other coffeeshop save for one or two have on offer out of the water. Also, on Fridays and Saturdays they've got a promotion of 3x5€ for 12.50€, or alternatively 1x5€ + 1x 12.50€ for 15€. You save the same 2.50€, but the big bags I find nicer, especially when the whole 1.3g or so is one big bud.

Music changes by the budtender in charge, but it varies from classic rock, psychedelic rock, occasional hard rock/metal, psychedelic trance/psybient, obligatory reggae, trip hop... I've heard the greatest variety (and also the most stoner-friendly) music here in all of Groningen.
HIGHLY recommended! Also, ask the budtenders - they won't talk shit, I was told by a budtender not to buy a big bag of something (that would have brought in 12,5€) but two small bags of another instead (10€) because he'd seen me smoke before and the one in the big bag I might find a bit lacking in potency.

Some strains that have shown up recently/often:
WEED
Afghan x K2
Snow White
White Widow
Bubble Gum
Silver Haze
Amnesia Haze
Choco Haze
Shiva
Shiva Shanti
Orange Bud
Big Buddha Cheese
Sour Diesel
HASH
Moroccan
Yellow Lebanese
Afghan Kabul (10€ Afghan, nothing like the 5€ squidge, has a stoney, dopey high reminiscent of the royal family of dark hashes: Temple Balls, Malana Cream)
Temple Balls
Zero Zero