^ After using the Pocket Operators, I love seeing other devices that utilize the same little buttons 
Not sure how you would pair it with the OP-1 other than just straight sampling it, but the new Moog Model15 app is stellar. It may have a steep-ish learning curve if you are not familiar with analog synths, but man, the thing is so well designed and sounds amazing.
I’m in love with the System 15 app! Very intuitive and much easier to get around then I expected. With a midi to usb converter and the camera attachment for Ipad you can easily control the Moog beast with OP. It even supports multi-expressive controlers like Roli Seaboard and Continuum Fingerboard.
If does look/sound ace. But my aging iPad 2 will not support it 
On the wish list for now
Just found quite a few iPad/op1 youtubes by zulumonk. This is the kind of spirit/thought process for starting the thread. Maybe of interest to you guys as well.
^ he did one with Auxy as well. The iPad version is free and pretty great for midi sequencing the OP-1.
Best app i think is the TuneIn Radio app… one of the best aspects of the OP-1 is the radio… but the FM reception is sometimes limited. With that app you have a massive worldwide samplescape.
I don’t really think you need any additional synthesiser apps or sequencer stuff - there is enough in there already for creation + the tape tools and resampling