Ó Sé subsequently joined the Kerry under-21 team in 1994. While it was an unsuccessful year for the team in that grade, Ó Sé's nailed down his position at midfield and the following year lined out in his first Munster under-21 final. Opponents Waterford were inflicted with a 1–21 to 2–5 trouncing, giving Ó Sé a Munster winners' medal in the under-21 grade. Kerry later qualified to play Mayo in the All-Ireland final. After the highs of the Munster final victory, Kerry were held to a draw and a replay was required. That game was also a close affair, however, Kerry's goal-scoring ability proved the key. A 3–10 to 1–12 score line saw Kerry claim the victory and gave Ó Sé an All-Ireland winners' medal with the Kerry under-21 team.