Ableton Link works well for syncing tempo between "link-enabled" apps/software on the same network. It's great for peer-to-peer jam sessions with multiple iOS devices and Ableton Live (although Live isn't necessary).
The OP-1 can get in on the fun too. Here's one way to do it:
1. Connect the OP-1 to an iOS device via the Apple Camera Connection kit.
2. Download a free iOS app called "Link to MIDI" by Alexandernaut (author of the awesome Fugue Machine app).
3. Open the Link to MIDI app. Press the "MIDI" button and select the OP-1, which should be available as an option under "MIDI destination."
4. On the OP-1, press the tempo button and turn the green controller until the middle section shows "Link" at the top, "Ext" in the middle, and "Sync" at the bottom.
Now the OP-1's tempo will be synced with the Link To MIDI app and any other link-enabled apps/software connected to the network. If you change the tempo on any of the apps you will see the tempo change on the OP-1.
I've attached an image of my setup.