Grrr..
I hate all these "Oxford says this, Oxford says that" rumours - that are founded on NOTHING. Oxford choose the best candidates that have applied - that is the only solid reason for admission.
As long as you have around 7A* (average GCSE grades from their candidates) and AAAA at AS Level and AAA at A2 Level, a good BMAT, some extra curriculars and some work experience, you should get to interview. (A good reference and personal statement go witout saying...)
After that, it is up to you.
Who knows - some people who did work experience since God was a boy may get rejected, they may not. Those who only did a week may also get in, they may also not. If you're a good enough candidate and will get AAA for A2 Level, you'll get in, end of.
So, no, that rumour is just that, a rumour - though having a good deal of work experience will not harm you.