Combining notre with on

So....when speaking or writing informally we use 'on' in place of 'nous', but then what word does one use for 'our'?

Should I write:

On cherche nos livres

or

On cherche ses livres

as an informal alternative to

Nous cherchons nos livres

????

It's «On cherche ses livres.» (i. e. on would use the third person singular possessive: sa/son/ses)