Dictionary.com uses two main sources:
BASED ON THE RANDOM HOUSE UNABRIDGED DICTIONARY, © RANDOM HOUSE, INC. 2022
COLLINS ENGLISH DICTIONARY - COMPLETE & UNABRIDGED 2012 DIGITAL EDITION
© WILLIAM COLLINS SONS & CO. LTD. 1979, 1986 © HARPERCOLLINS
PUBLISHERS 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
So it is basically similar to Google as an aggregator. Provided that quotes from that site here actually reference the ultimate source of the definition, I don't see a problem.
Random House publish Penguin, but I can't find any mention of any English dictionary on either website. Collins publishes a number of dictionaries, and more than one is available on its own website.
Just as with "I found this definition on Google," which is unacceptable because it doesn't give credit to the people who actually did the work defining the word, "I found this definition on Dictionary.com" won't be acceptable either. The actual source needs to be cited: "provide the name of the original author".
OED continues to be available to those who have access.