There is now a solution that allows hub to hub integration between SmartThings and openHAB. It connects to the openHAB MQTT binding. I did get it successfully installed on a Raspberry Pi 3.
I am a former SmartThings user who migrated to openHAB over primarily reliability concerns. However, I do see two primary use cases:
1: A SmartThings user can allow openHAB to control their SmartThings setup, and experiment with openHAB sitemaps and rules before committing fully to a change.
2: Control zigbee devices (at least until the openHAB zigbee binding is ready for prime time).
Here’s the relevant links. Note the instructions are written for Home Assistant, but you can refer to the MQTT binding instructions if you need guidance for setting up your items.