For what is or is not an English word:
Consider the current version of the North American Scrabble® Players Association's Official Tournament and Club Word List and Long List as influential (and authoritative for Scrabble® Tournament play) on the question of whether or not a word of 15 letters or less is an American English word. Outside North America, consider the World English-language Scrabble® Players' Assocation's official list, called the Collins Scrabble Words.
For dictionaries considered authoritative:
Consider the dictionaries used to compile the Scrabble® lists as very influential overall. See also this answer at US Equivalent to the Oxford English Dictionary which gives a different list.
For questions of Style and Usage:
Several organizations publish "style guides". For academic writing, the most influential are The MLA Handbook for academic writing at the high-school and undergraduate level, The MLA Style Manual for more advanced scholarly writing, The AP Stylebook for news and journalism, and The Chicago Manual of Style for general purposes. Also, The American Heritage Dictionary is notable for its use of a "Usage Panel" of 200 "prominent users of the language" which provides some of the best guidance available for what is considered best usage of words and word forms among controversial alternatives.
While there is no offical regulatory body for the English language, one influential regulatory body for American English is the North American Scrabble® Players Association. They provide a list of words acceptable for use in Scrabble® play, and thus judge whether or not a word is a word.
To be fair, they acknowledge that their list is not a complete list of English words, excluding words no longer in use among other things. Also, because of the nature of the Scrabble® game, they do not address words longer than 15 letters. Still, this is, at least in my mind, the most authoritative list of American English words there is. Which is not to say that OED is wrong if they include a word not on that list, but rather that if a word is not on that list, I would not consider it a current American English word.
A few things to note about the Scrabble® lists:
- The Official Scrabble Players' Dictionary (OSPD) is meant for use in children's tournaments, and therefore excludes offensive words such as "asshole". It is not a complete list even by Scrabble® standards. As of this writing, the current version is OSPD4.
- The Official Tournament and Club Word List (OWL) only includes 2-9 letter words. As of this writing, the current version is OWL2, with the next version expected in 2014. It is also focused on usage in the United States.
- The Long List is a supplement to OWL2. It contains 10-15 letter words.
- The Collins Scrabble Words list (CSW) is a superset of OWL2 that adds words (and spellings) used outside of the US. The current version is CSW12
At the moment, you can access electronic versions of these lists via the free study program Zyzzyva. The lists from Zyzzyva include cursory definitions of one sense of the word. The definitions are provided only to satisfy basic curiosity about the word, not to be in any way comprehensive.
On the subject of authorities, it is worth noting what authorities were used to compile the Scrabble® word lists. OWL2 was compiled with reference to:
- American Heritage College Dictionary (4th edition),
- Merriam-Webster’s Collegiate Dictionary (11th edition, 2003 printing)
- Random House Webster’s College Dictionary (2nd revised and updated edition, 2000)
- Webster's New World College Dictionary (4th edition)
The Long List is published by Merriam-Webster, so it is perhaps not as surprising that it was based on Merriam-Webster’s Collegiate Dictionary, 10th edition
The CSW adds words from Collins and Chambers dictionaries to the current version of the OWL.