I saw justin in a video recently singing one time live and this is what his guitarist 
playing...i'm not too sure what some of the chords are called... so i looked up bar chords 
google. sorry if they arent the right ones, but when i play it it is 100% accurate. cool :)

Cm
Ab
Bb
Eb

Cm          Ab                         Bb         Eb
Me plus you, I'ma tell you one time
Me plus you, I'ma tell you one time
Me plus you, I'ma tell you one time
One time, one time

Cm                                        Ab
When I met you girl my heart went knock knock
Bb                                         Eb
Now them butterflies in my stomach won't stop stop
Cm                                        Ab
And even though it's a struggle love is all we got
Bb                                        Eb
And we gon' keep keep climbing to the mountain top

(and so on)

Your world is my world
And my fight is your fight
My breath is your breath
And your heart

And girl you're my one love, my one heart
My one life for sure
Let me tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

And I'ma be your one guy
You'll be my #1 girl
Always making time for you
I'ma tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

You look so deep, you know that it humbles me
You're by my side, them troubles them not trouble me
Many have called but the chosen is you
Whatever you want shawty I'll give it to you

Your world is my world
And my fight is your fight
My breath is your breath
And your heart

And girl you're my one love, my one heart
My one life for sure
Let me tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

And I'ma be your one guy
You'll be my #1 girl
Always making time for you
I'ma tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

Shawty right there
She's got everything I need
And I'ma tell her one time
Give you everything you need down to my last dime

She makes me happy
I know where I'll be
Right by your side
'Cause she is the one

And girl you're my one love, my one heart
My one life for sure
Let me tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

And I'ma be your one guy
You'll be my #1 girl
Always making time for you
I'ma tell you one time
(Girl, I love, girl I love you)
I'ma tell you one time
(Girl, I love, girl I love you)

Me plus you, I'ma tell you one time
Me plus you, I'ma tell you one time
Me plus you, I'ma tell you one time
One time, one time