How is "plenty" a pronoun in "plenty of time"?

The Oxford Dictionaries list "plenty" as a pronoun. Example sentences include:

I would have plenty of time to get home before my parents arrived

There are shops in plenty

But pronoun by definition is a word that substitutes for a noun or noun phrase. How is "plenty" a pronoun in these examples? What noun/noun phrase does it substitute for? In contrast, Merriam Webster lists it as noun, not pronoun. I think in the sentence You will have plenty to draw from "plenty" is a pronoun, but it being a pronoun doesn't make sense in the two sentences above. How is the usage of "plenty" in those sentences different from that of these ones:

I need a large amount of money.

There was food and drink in abundance.

Both "amount" and "abundance" are listed as noun in dictionaries.

The Original Poster is correct to be dubious about the parts of speech given by Oxford Dictionaries Online. In fact, one should be dubious about the part of speech information given in dictionaries, as a general rule. Dictionaries are wildly out of date regarding grammatical information, and effectively ignore all of the developents in this area of linguistc science since about 1920. (There are good reasons for this as described in the linked-to post)

But then again, why look up grammar information in a dictionary? Dictionaries are great at lexicography, not grammar. It's best to look up grammar information in a modern vetted grammar book!

The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) gives the word plenty as a noun, not a pronoun. More specifically, they describe it as a number transparent quantificational noun.

They further write:

Plenty is singular in form but does not admit any determiner or modifier: plenty of butter/friends, not * a remarkable plenty of butter/friends . (p.350)

The Original Poster's analysis and objections are generally very astute, apart from that even in I have plenty to choose from, the word plenty is still a noun. OP right, OD wrong!