The third-year scholar got his name on the scoresheet in the 2-2 EFL Trophy draw at the Kassam Stadium.
It represented a memorable moment for the Gosport lad, who wheeled away in celebration after cancelling out Matty Taylor's opener.
Lethbridge’s effort helped the Blues ensure they finished top of Southern Group B, after claiming a bonus point for winning the penalty shootout 5-4.
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Yet the forward, who’s been on loan at Bognor this season, had no idea he’d be involved until a day before the game.
And he took his opportunity for Kenny Jackett’s side.
Lethbridge said: ‘I just didn’t think I would be playing, so when I saw my name on the team sheet then I was taken by surprise.
‘A lot of the other youngsters have been playing in the competition and I hadn’t been selected.
‘So when I saw my name on the team sheet, it felt good.
‘I only found out on Monday. When I went into training, my name was on the board in the morning, it was took me.
‘I just thought “wow” because I had been out injured, played one game for Portsmouth in pre-season and a few at Bognor.’
Lethbridge made just his third senior outing against Oxford.
Leading Pompey’s attack, he came up against two regular League One centre-backs in Rob Dickie and Elliott Moore.
It was a challenge Lethbridge had to acclimatise to and he feels it will help his promising progress.
He added: ‘It was a lot more physical, the defenders are stronger and go and attack the ball a lot more.
‘They just don’t want to lose the battle.
‘Southend last season was a harder game for me. I couldn’t really get into the game but I felt I got a lot more into it against Oxford.
‘I’m adapting to playing against different centre-halves and exploiting their weaknesses.
‘I thought I should have perhaps got in behind a bit more because their centre-halves weren’t the quickest.’