There is no such thing as an "authoritative dictionary" for the English language. See prescriptive versus descriptive linguistics/lexicology. If the Oxford dinosaurs have screwed up, whatever with them. In some cases there is no readily articulable rule, but native English speakers nonetheless tend to converge on a common usage for new terms in which those using the "lesser" rendering end up changing their minds upon thoughtful questioning.

"Open source", as Luke says, is the only possible correct rendering. Compare, e.g., in contradistinction, things like "potato-based" and "cat-friendly", which are *adj. + past part. -> adj.* and *n. + adj. -> adj.*, respectively. "Open source" is *adj. + n. -> adj.*, and treated differently, though in my present drunken state I am challenged to find a parallel form of an unhyphenated compound adjective.

I'm not volunteering for the role, but I've done about 4 years of professional SK/CZ->EN translation as well as lots of EN correction. The EN text, if it's to be accurate and a solid source for translation into other languages, should have the attention of a native English-speaking *(adj. + [n.-pres. part.] -> adj.[!])* professional linguist/translator/copyeditor, or at least someone thoroughly versed in the field.

Also, in the industry, "proofreader" leans toward a lesser competency than "copyeditor" or the others, concerning itself principally with last-minute remedies, at which point layout is committed, for typos and punctuation just before commitment to a print run, whereas the others are free to make wordsmithing changes to usage, idiom, flow etc. Therefore, layout-insensitive *([n.prep.]-adj. -> adj.[!!!])* web text generally demands a copyeditor but not a proofreader, according to industry-standard *(o.m.g.-not-again[!])* terminology.