au premier / à la première
I'll speak to him at the first opportunity.
Je lui parlerai dès que possible.
I will see him at the first opportunity.
Je le verrai dès que possible.
But because I know he'd be hurt at the first weird glance.
Mais parce que je sais qu’il serait blessé au moindre regard bizarre.
— Perfect Strangers (2016)
I will do it at the first opportunity.
Je le ferai à la première occasion.