Looks like Olivier Martinez is lighting up to take the edge off amid his custody battle with Halle Berry ... because cameras caught him smoking outside court.

TMZ obtained a photo of Halle's ex smoking Monday in the midst of their contentious custody battle over their 13-year-old son Maceo.

Olivier's rocking shades as he smokes, and is sporting a very different expression than his actress ex-wife was as she left the courthouse that day.

As we previously reported ... Halle seemed happy heading into court and was still all smiles as she left hand-in-hand with boyfriend Van Hunt.

Halle and Olivier's son Maceo gave his sealed testimony Monday -- and the judge ordered both parents out of the room while he did.

As you know, the exes are locked in a legal battle after Olivier accused Halle of attacking and choking their son while arguing over a VR game he'd been playing.

A judge previously granted Olivier a temporary restraining order against Halle ... which limited how much contact she's able to have with Maceo.

Olivier is fighting for sole custody of their kid ... but Halle wants the court to restore their previous arrangement.

Halle has denied the accusations against her.