Oxford needs to look up the word "slang."

[Burly Brute]

I'm not really sure what the problem is, The Oxford Dictionaries are simply that, a large list of words that are used in the English language, they don't suggest which words to use, merely which words have been used.To be realistic they will forever be putting words in which are not absolutely current, as any word has to have some use before many people know what it is.
English is one of the few languages which continually adds words to the canon, and doesn't remove them.
To me as a British English speaker, but not an native of England, I enjoy the richness and variety that English has, and am very glad that some people do take the trouble to write it down

for a current dictionary of slang try this Urban

But "PROLLY" is just Probably, or "SAMMICH" is just sandwich, but spelled like that because people are either dumb/lazy. So they aren't technical words to begin with, they are just slang terms which should be religated as such!