Casimero vs. Canchila

On December 19, 2009, in Managua, Nicaragua, Casimero won the then-vacant interim WBO light flyweight title, by defeating former interim WBA Light Flyweight champion César Canchila of Colombia. Canchila had a record of 28–2 coming into the bout, with his last loss coming from current WBA light flyweight world champion Giovanni Segura, whom he previously defeated for the interim title. Casimero gave a spectacular performance, the fight being his first outside his home country. He was able to knockdown Canchila multiple times until the referee finally stopped the bout in the 11th round of the scheduled 12 round interim title fight.^[6] This win can give Casimero a title shot against Iván Calderón of Puerto Rico, who previously defeated Filipino boxer Rodel Mayol.^[7]

Casimero vs. Hirales

Casimero fought Ramón García Hirales on July 3, 2010, for his first defense of the interim WBO light flyweight title but lost by split decision.

Casimero vs. Mthalane

Casimero got his first shot at a world title when he challenged Moruti Mthalane of South Africa for the IBF flyweight title on March 26, 2011.^[8] He was, however, stopped by the South African in five rounds, therefore marking his second career loss.^[9]

Casimero vs. Lazarte

On February 11, 2012, Casimero, and members of his team were kicked and assaulted when fans in Mar del Plata hurled chairs and stormed the ring following his 10th-round knockout of local Argentine fighter Luis Alberto Lazarte for the IBF interim junior flyweight championship.^[10] After the riot, police escorted Casimero and his team to their hotel and provided protection for them. Lazarte later visited to apologize.^[10] The Philippines has filed a diplomatic protest to the Argentine government after Argentine fans attacked Casimero in the ring after winning the title bout.^[10][11] Philippine Foreign Affairs spokesman Raul Hernandez stated that the Philippine Embassy in Buenos Aires filed a protest with Argentina's Ministry of Foreign Affairs and is awaiting an explanation.^[10] Lazarte was banned by IBF for threat, sparking a ring riot.^[11]

On July 19, 2012, Casimero was upgraded to the status of full IBF junior flyweight champion because of the inability of Ulises Solis to defend his title on or before October 30.^[12]

Casimero vs. Guevara

On his first defense of the IBF junior flyweight title, Casimero faced Pedro Guevara of Mexico on August 4, 2012. He scored a knockdown in the opening round and fought to a split decision victory.^[13]

He successfully made a second defense of his title against WBC Latino light flyweight champion Luis Alberto Rios on March 16, 2013.^[14] His third defense was against Felipe Salguero on October 26, 2013 and he won by eleventh-round knockout.^[15]

Casimero vs. Ruenroeng I & II

Following his win against Mauricio Fuentes and Armando Santos, Casimero moved back up to the flyweight division. On June 27, 2015, he lost his return to flyweight bout via unanimous decision against IBF champion Amnat Ruenroeng in a controversial match.^[16] A rematch on May 25, 2016, Casimero challenged Ruenroeng again for the IBF flyweight title and this time he knocked Ruenreong out with a vicious left hook to the body in the fourth round to become a two-division world champion.^[17]

Casimero vs. Edwards

In his second title defence, Casimero faced undefeated Charlie Edwards, on September 10, 2016 on the undercard of Gennady Golovkin vs. Kell Brook at The O2 Arena in London, England. Casimero was on the offensive for most of the fight, as Edwards was unable to deal with Casimero's ring craft. Casimero upped the pace in the tenth round, and dropped Edwards with a left hook. Edwards beat the count, but as soon as he got up, Casimero unleashed a flurry of punches towards the challenger, which prompted the referee to end the fight immediately.^[18]

Casimero vs. Tete

On 30 November 2019, Casimero challenged Zolani Tete for his WBO bantamweight title in Birmingham, England. Despite Tete being the favorite for the fight, Casimero scored a third-round knockout over the long-reigning champion to capture his WBO bantamweight title. The victory meant that Casimero became a three-division world champion.^[23]

Casimero vs. Micah

Casimero have scheduled to face undefeated unified bantamweight champion Naoya Inoue in a unification clash on April 25, 2020, but the COVID-19 pandemic caused the fight to be cancelled.^[24] He instead faced the undefeated Duke Micah, ranked #11 by the WBO, in his first WBO bantamweight title defense on September 26, 2020 on the undercard of The Charlos vs. Dereyvanchenko and Rosario. Casimero recorded another stoppage win, again in the third round, to retain his WBO belt.^[25]

Casimero vs. Rigondeaux

It was announced on June 19, 2021 that WBC champion Nonito Donaire would replace Rigondeaux,^[26] but when the fight between Casimero and Donaire fell apart over insults directed at Donaire's wife,^[27] Rigondeaux stepped back in to fight Casimero as had been originally planned.^[28] Casimero faced two-time Olympic gold medalist Guillermo Rigondeaux on August 14, 2021.^[29] In a low-key affair that saw the CompuBox record broken for fewest combined landed punches in a 12-round fight, Casimero pressured his opponent all night, fighting on the front foot and throwing more punches than Rigondeaux, who was reluctant to engage. Casimero was rewarded with a split decision victory, with scores of 117–111 and 116–112 in his favor, and 115–113 in favor of Rigondeaux.^[30]

In his post-fight interview, Casimero criticized his opponent's extremely passive gameplan, saying "I'm focused on [the] knockout, but Rigondeaux always runs. Rigondeaux just always runs. No fighting." He continued by reaffirming that it is his intention to unify the bantamweight division, targeting the division's other titleholders, Nonito Donaire and Naoya Inoue. Casimero concluded his interview by issuing the middle finger to Inoue.^[31]

Cancelled bout vs. Paul Butler

Casimero had been scheduled to fight his mandatory challenger and former IBF bantamweight champion Paul Butler on December 11, 2021 at Coca-Cola Arena in Dubai, United Arab Emirates. Casimero withdrew before the weigh-ins after being rushed to the hospital due to viral gastritis.^[32] The bout had been rescheduled on April 22, 2022 at M&S Bank Arena in Liverpool, England but Casimero was not permitted to fight by the British Boxing Board of Control (BBBofC) due to medical guidelines violation by using a sauna to cut weight "in close proximity" to the fight.^[33][34] Casimero's late replacement, fellow Filipino Jonas Sultan, lost to Butler by unanimous decision, making Butler the interim champion.^[35] On May 4, the WBO stripped Casimero of the title, with Butler supplanting him.^[36]