A complication here is some confusion over how you define acronym. Some say that it is any string of initial letters and a synonym of initialism. Others say that an acronym is a word made of initial letters (like Nato) and it is an initialism if you pronounce each letter separately as in HTML.
This is not universal though:
Definition of acronym
an abbreviation formed from the initial letters of other words and pronounced as a word (e.g. ASCII, NASA). Compare with initialism.
Definition of initialism
an abbreviation consisting of initial letters pronounced separately (e.g. BBC). Compare with acronym.
While this seems clear enough, compare with Dictionary.com:
a word formed from the initial letters or groups of letters of words in a set phrase or series of words, as Wac from Women's Army Corps, OPEC from Organization of Petroleum Exporting Countries, or loran from long-range navigation.
a name or term formed from the initial letters of a group of words and pronounced as a separate word, as NATO for North Atlantic Treaty organization; an acronym.
a set of initials representing a name, organization, or the like, with each letter pronounced separately, as FBI for Federal Bureau of Investigation.
Contrast this with OED:
A group of initial letters used as an abbreviation for a name or expression, each letter or part being pronounced separately; an initialism (such as ATM, TLS).
A word formed from the initial letters of other words or (occas.) from the initial parts of syllables taken from other words, the whole being pronounced as a single word (such as NATO, RADA).
So, OED's acronym is the same as Dictionary.com's initialism. For initialism OED says:
The use of initials; a significative group of initial letters. Now spec. a group of initial letters used as an abbreviation for a name or expression, each letter or part being pronounced separately (contrasted with acronym n.).
Summary: ODO says that an acronym is a word and an initialism is letters. Dictionary.com says that an acronym is a word but an initialism can be a word or letters. OED says that an acronym is a word or letters, but an initialism can be only letters.