のは僕だけ

I'm the only one who...

→ の
phrase Rare #29,014
I'm the only one who... Used after a clause to say that the speaker alone is the person who does or experiences something. 僕 is a casual masculine way to say “I,” and だけ means “only.”
JA: この問題に気づいたのは僕だけだった。
EN: I was the only one who noticed this problem.

そう思ってるのは僕だけじゃないよ

I’m not the only one who thinks so.

残ってるのは僕だけ

I'm the only one left.

— ライフ・イズ・ビューティフル (1997)

フランス語を話せないのは僕だけだった

I was the only one who couldn't speak French.

ここでフランス語を知らないのは僕だけです

I'm the only one here who doesn't know French.

が出たのは僕だけ?

Am I the only one who got a fever?

— Hoje Eu Quero Voltar Sozinho (2014)