The 21-year-old has joined the Blues for a club-record £24m + add-ons fee from French club Toulouse.
Hull City, who, like Ipswich, have just been promoted to the Premier League were among those seriously interested in the front man.
O’Neil, who saw Emersonn score against his Strasbourg side in May, said: "We’re delighted to welcome Emersonn to the club.
“He is a young striker with real ability and potential who has had plenty of interest this summer, so for him to choose Ipswich Town is something we are very proud of.
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“I know he is excited to be playing in the Premier League and we’re looking forward to having him with us as we prepare for the new season.”
Upon signing, Emersonn said: “I am very proud to sign for Ipswich Town.
“Playing in the Premier League is something I have always dreamed of and I know I have picked the perfect club to do that with.
“I will give everything for the team every time I play and I can’t wait to get started and meet my new teammates as well as the supporters.”