Just 343 votes separated outgoing councillor Merepeka Raukawa-Tait (4716) and O'Brien.
Raukawa-Tait previously told the Rotorua Daily Post she was "fatalistic" about the results.
"It's five minutes of disappointment and on with life.
"It took me three goes to get on there in the first place, and I never take it personally. I have been privileged to serve 11 years, and it's not as if I don't have other work to do."
Tapsell previously told Local Democracy Reporting: "It's very important that we have a functional council and work together so I'll be asking them what their top priorities are."
Tapsell, a three-term Rotorua Lakes councillor, announced her mayoral bid in May on a platform of stopping "the spend", building a safer community and "smart" investment.
Declaration of result
Mayoralty
Tapsell, Tania - 7362
Sandford, Ben - 4070
Tabuteau, Fletcher - 4043
Kumar, Raj - 3600
Macpherson, Reynold - 3561
Sears, Rania - 377
Ananda, Kalaadevi - 289
Informal - 29
Blank voting papers - 205
Rotorua General Ward (6 vacancies)
Wang, Fisher - 8110
Kai Fong, Sandra - 8045
Brown, Gregg - 6785
Lee, Robert - 5539
Paterson, Don - 5160
O'Brien, Conan - 5059
Raukawa-Tait, Merepeka - 4716
Gray, Ryan - 4713
Davis, Brendan - 4325
Hosking, Trish - 4209
Chater, Nick - 4089
Pinder, Steve - 4018
Yates, Mercia - 3512
Chetty, John - 3469
Sparke, Kaya - 3229
Te Kowhai, Rawiri - 3074
Tuakana-Jones, Grace - 2498
Black, Kariana - 2417
Sear, Rania - 1994
Informal - 43
Blank voting papers - 244
Rotorua Māori Ward (3 vacancies)
Kereopa, Lani - 2158
Maxwell, Trevor Horowaewae - 1906
Waru, Rawiri - 1766
Tapiata, Hana - 1618
Biasiny-Tule, Potaua - 1608
Hataraka, William - 512
Informal - 1
Blank voting papers - 199
Rotorua Rural Ward (1 vacancy)
Barker, Karen - 1565
Trumper, Shirley - 796
Informal - 0
Blank voting papers - 90
The voter return was 46.46%, being 22,920 votes, excluding special votes.
