Any help on finding the biggest freely available English corpus that can be used on research?
So far I have found OANC with 15 M words.
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Sketch Engine, a corpus manager and text analysis software, provides a few corpora with open access for research on https://app.sketchengine.eu/#open The largest English corpus (freely available) is ACL Anthology Reference Corpus with 62 million words.
On the other hand, you can try 30-day free trial of Sketch Engine and search one of the biggest English corpus which currently exists with over 35 billion words, see more at https://www.sketchengine.eu/timestamped-english-corpus/
Can't beat the Global Web-Based English Corpus proposed by robert---but here are is another big one:
A Wikipedia dump is also huge ...
Westbury labs provides a ~1 billion word Wikipedia dump of all articles with greater than 1000 words from 2010: http://www.psych.ualberta.ca/~westburylab/downloads/westburylab.wikicorp.download.html
BYU has a larger dump (1.9 bn) from 2014, but it's not available for download.
The COBUILD corpus (18M tokens) is available through WebCelex, if the arcane user interface isn't a deal-breaker. It's valuable more for its extensive manual annotations than its size, with quite a lot of morphological and phonological information available.
(It's smaller than most of the others listed here, but seems worth mentioning, since it's larger than the OANC mentioned in the question and is well-annotated.)