Where can I find the correct idioms?

[RaffaeleM39]

I was doing the following MH GMAT question

According to the international investment memorandum recently signed in Geneva, France is one of the 4 European nations planning to provide fewer tax incentives for foreign investment in production of heavy industrial machinery.

A. planning to provide fewer

B. planning to provide less

C. planning on providing fewer

D. which is planning on providing fewer

E. that is planning to provide less

The explanation of answer C says that

(C) This answer choice uses the unidiomatic construction “to plan on.” The appropriate idiom is “to plan to.”

But the Oxford English Dictionary says that "to plan on" is acceptable. The following example is given:

We hadn't planned on going anywhere this evening.

1. Where can I find the correct idioms to use in GMAT?
2. Why do you say the idiom is incorrect if the dictionary says it is correct?

Thanks

EDIT: The Merriam-Webster dictionary uses "to plan on" as well

to have a specified intention —used with on plans on going

EDIT 2: In the "Sentence Correction" strategy book, the idiom "to plan" is not present

[Sage Pearce-Higgins]

There are two issues going on here. First is that there are many idioms that are considered unacceptable in official GMAT problems. GMAT is a little arbitrary about this, but tends to stick to slightly old-fashioned, formal ways of saying things. Dictionaries, on the other hand, are updated to include modern and colloquial usage of language. Therefore dictionaries often contain idioms that GMAT considers unacceptable. Our idiom list is intended to bridge the gap, but we gather our information from GMAT problems as they're released each year.

Sure, I know that the problem you're discussing is from one of our practice tests, and there's a second point here. When we say that an idiom is unacceptable, we don't mean unacceptable in any circumstances, just in that particular situation. So, to say 'I'm planning on going out this evening.' is fine; it makes the plan sound a bit informal, perhaps not finalized, and a little vague. However, to say that 'The army is planning on attacking at dawn.' is inappropriate, as clearly the army has planned a bit more thoroughly for its attack than I have for my evening out. So, for more formal or organized situations, we'd use the idiom 'plan to'.

Unfortunately these nuances of meaning are too numerous to list. Getting a deeper understanding by considering how you use language, making your own examples, and learning from your errors is the way to improve here.