You should be able to go either way. At my last job I had one of my techs setup about 20 thin clients for a group of interns we had doing data entry. We wanted to save as much money as possible so we went for some thin clients that pxe booted from a linux server and then they ran open office so I didn't have to pay for the MS office license for them.
They worked on spreadsheets so I am pretty sure that the end product was visible in xls. I don't think we had them save to csv or anything first either.
It might be different with things like powerpoint to openoffice's equivalent, we didn't have them work on that type of stuff.
For the price (free... can't beat that) its a prettty darn good product.
I'm not disputing what CardFan67 said, as he probably has more direct experience with the product than I did, and we did most work with going back and forth with xls spreadsheets, not .doc stuff. One thing openoffice was good for with .doc, was we kept running into people corrupting doc files since there is a bug in office xp that when using markup, sometimes the file would become unreadable except as straight text and then all the formatting was lost. Openoffice could open and fix the file. Pretty cool. Then we just pushed out all the office xp service packs and the problem went away. But in the meantime, openoffice was a lifesaver.