Your post title mentions OpenStudio specifically, which I do not know of certifications for that specific tool. However, there are ways to achieve certification in general energy modeling practices, which you can certainly apply to OpenStudio projects.

General energy modeling certification

• ASHRAE Building Energy Modeling Professional (BEMP) - pass an exam
• AEE Building Energy Simulation Analyst (BESA) - requires that you attend a seminar as well as pass an exam

While not resulting in a license or certification for OpenStudio, there are multiple options for you to receive training.

OpenStudio training providers

• Performance Systems Development (PSD)
• EnergyModels.com
• The Energy Simulation Academy (TESA)
• EffiBEM
• Big Ladder Software - DISCLAIMER: I am the lead trainer at Big Ladder Software

There are also options for you to teach yourself how to use OpenStudio.

Teach yourself how to use OpenStudio

• Building Energy Modeling with OpenStudio: A Practical Guide for Students and Professionals
• Free NREL OpenStudio tutorial videos - note that these videos are ~8 years old and slightly outdated
• Free Helix Energy Partners OpenStudio tutorial videos - tutorial videos from the last year or so