| Billy O'Brien | p | GAL - ING | 2012/13 | 0 | 0 | Manchester City U19 | | | |
| Lee O'Connor | c | IRL | 2018/19 | 2 | 0 | Manchester United U19 | | | |
| Nosa Obaretin | d | ITA - NIG | 2020/21 | 6 | 0 | Napoli U19, Milan U19 | | | |
| Daniel Obbekjær | d | DAN | 2020/21 | 0 | 0 | Spal U19 | | | |
| Marek Obdržálek | p | RCE | 2023/24 | 0 | 0 | Slavia Praga U19 | | | |
| Adam Obert | d | SLV | 2020/21 | 4 | 0 | Cagliari U19, Sampdoria U19 | | | |
| Federico Oberti | c | ITA | 2003/04 | 2 | 0 | Albino Leffe U19 | | | |
| Oberti | a | ARG | 1989/90 | 0 | 0 | Newell's Old Boys U19 | | | |
| Obi | a | SVI | 2024/25 | 0 | 0 | Basilea U19 | | | |
| Pedro Obiang | c | GUE - SPA | 2009/10 | 3 | 0 | Sampdoria U19 | | | |
| Nwankwo Obiorah | c | NIG | 2011/12 | 1 | 0 | Parma U19 | | | |
| Matteo Obleac | d | MOL - ROM | 2025/26 | 2 | 0 | Frosinone U19 | | | |
| Justin Oboavwoduo | a | ING - NIG | 2024/25 | 1 | 0 | Manchester City U19 | | | |
| Relja Obrić | d | SLO - SER | 2023/24 | 1 | 0 | Atalanta U19 | | | |
| Ocaro | c | ITA | 2006/07 | 1 | 0 | Pro Sesto U19 | | | |
| Tommaso Occhi | d | ITA | 2024/25 | 1 | 0 | Spal U19 | | | |
| Simone Ocello | a | ITA | 2003/04 | 1 | 1 | Fiorentina U19 | | | |
| Tommaso Odasso | c | ITA | 2016/17 | 0 | 0 | Udinese U19, Genoa U19, Carpi U19 | | | |
| Brian Oddei | a | ITA - GHA | 2018/19 | 2 | 0 | Sassuolo U19 | | | |
| Riccardo Oddi | d | ITA | 2019/20 | 2 | 0 | Milan U19 | | | |
| Paul Odendahl | d | GER | 2015/16 | 0 | 0 | Borussia Mönchengladbach U19 | | | |
| Martino Odero | d | ITA | 2025/26 | 2 | 0 | Genoa U19 | | | |
| Jens Odgaard | a | DAN | 2017/18 | 1 | 0 | Inter U19 | | | |
| Jannik Oettinger | a | GER | 2024/25 | 0 | 0 | Stoccarda U19 | | | |
| Daniel Offredi | p | ITA | 2007/08 | 2 | -5 | Milan U19 | | | |
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