Japan midfielder Hiroshi Kiyotake struck twice, including the winner nine minutes from time, as the home side came from behind three times to complete a late victory.
Twice champions Al Ittihad beat visiting K-League champions FC Seoul 3-1, 10-man Sepahan held on for a 1-0 win over Al Sadd and Suwon Samsung Bluewings salvaged a 1-1 home draw against last year's finalists Zobahan in the other quarters.
Jeonbuk took the lead in the sixth minute against Osaka when forward Lee Dong-gook rifled home from close-range but Ryuji Bando equalised for the home side in the 29th minute.
Lee scored his second in first half stoppage-time to put the Koreans ahead at the break but Kiyotake levelled by nodding home from a Noriyuki Sakemoto corner in the 56th minute.
Jeonbuk went ahead again when Cho Sung-hwan chested the ball in from a corner only for Kim Bo-kyung to equalise for Osaka in the 64th with a penalty given for handball.
An exhausted Jeonbuk then left Kiyotake unmarked and the attacking midfielder scored his second goal from a Sakemoto corner to seal a brilliant comeback victory for Osaka.
"Our players showed great concentration at a crucial moment and that produced our winning goal," said Osaka manager Levir Culpi whose team visit South Korea for the return on September 27.
Second-half goals from Osama Al Harbi and Geraldo Wendel gave Saudi Arabian club Al Ittihad a two-goal advantage over FC Seoul after Mohammed Noor gave them the lead on the stroke of half-time at the Prince Abdullah Al Faisal Stadium in Jedda.
The second leg is at Seoul's World Cup stadium on September 27.
SENT OFF
Sepahan took a 12th-minute lead over Al Sadd through Ibrahimi's low 25-metre shot and held on despite losing midfielder Akbar Imani before the hour after he was sent off for a second booking in the Iranian city of Isfahan.
The return leg will be played on September 28 in Doha.
Suwon had Park Hyun-beom to thank for their 66th minute equaliser against Zobahan who led through Mohammad Ghazi.
The home side dictated the early play at the Suwon World Cup Stadium but could not cash in on their chances while their Iranian opponents relied on counter-attacks.
In a barren first half, Suwon's Stevica Ristikj had two chances but his headers lacked direction.
Zobahan's Brazilian forward Igor Castro could have broken the deadlock in the first half but his low shot was parried for a corner by the alert Jung Sung-ryong.
However, the home side had no answer to the counter-attack that Castro orchestrated in the 56th minute. The 30-year-old forward darted along the left flank and unleashed a defence-splitting pass for Ghazi to tap home from close-range.
Park Hyun-beom calmed Suwon's nerves with the equaliser 10 minutes later though, volleying home a Park Jung-jin cross.
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