Hi.Looks like the flowers weren't pollinated.So,probably not self-fertile wich means you need a 2nd plant,same species but different clone.(These plants are easily (and usually)multiplied from parts of the stems (stem-cuttings)).Hope this helps.T.

Well that explains why I could not find anything similar enough before. I believe it may be an echinocereus viridiflorus then.

Hm, I see that most Echinopsis oxygona cacti on Google have very short (or virtually no) thorns, but perhaps this is a different subspecies or cultivar, anyways it definitely is an Echinopsis. Thank you very much :)

Gill is right with her id. More specific is Cereus peruvianus, just in case you wanted to know. It's skinny usually they're chubbier, but that doesn't matter:). If you want to give pieces away you can cut pieces that are about 6 pieces inches tall. Just mark what end is up. Let callus for a couple weeks and set on top of some mix. These guys you don't really have to baby:). Just no water or harsh sun till roots form. That could take a month or two. I got a little 4-5 inch cutting of this plant two years ago. The first year it didn't do much. Probably just working on growing strong roots. The next year is a different story, that thing has grown 16 inches! I find my plant to be an opportunistic grower. It put on most of it's growth in the late winter and spring, but that's when we get our rains. I'm telling you this because if you do give pieces away they may not do much the first year, but the second is a different story:).