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( Mark One ) UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE FISCAL YEAR ENDED DECEMBER 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission file number : 1-7933 IRELAND ( State or other jurisdiction of incorporation or organization ) Aon plc ( Exact name of registrant as specified in its charter ) Metropolitan Building , James Joyce Street , Dublin 1 , Ireland ( Address of principal executive offices ) ( Zip Code ) Title of Each Class Class A Ordinary Shares , $ 0.01 nominal value D01 KOY8 98-1539969 +353 1 266 6000 ( Registrant's Telephone Number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol AON Securities registered pursuant to Section 12 ( g ) of the Act : NONE Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes > No Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Exchange Act . Yes No Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes > No ( I.R.S. Employer Identification No. ) Name of Each Exchange on Which Registered New York Stock Exchange Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes > No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of “ large accelerated filer , ” “ accelerated filer , ” “ smaller reporting company , ” and “ emerging growth company ” in Rule 12b - 2 of the Exchange Act . Securities registered pursuant to Section 12 ( b ) of the Act : Large accelerated filer Accelerated filer Non - accelerated filer Smaller reporting company Emerging growth company 0 If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No As of June 30 , 2020 , the aggregate market value of the registrant's Class A Ordinary Shares held by non - affiliates of the registrant was $ 44,593,079,186 based on the closing sales price as reported on the New York Stock Exchange - Composite Transaction Listing . Number of the registrant's Class A Ordinary Shares of Aon plc , $ 0.01 nominal value , outstanding as of February 18 , 2021 : 225,984,346 . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's proxy statement for its 2021 Annual General Meeting of Shareholders are incorporated by reference in this report in response to Part III , Items 10 , 11 , 12 , 13 , and 14 .