OTG on ipad air 1st gen

Ah, no. Whilst the Lightning to USB3 Camera Adapter is compatible with your iPad, support for external USB storage is available with iPadOS13 or later.

iPadOS provides limited support for external Hard Disk storage.

An external HDD can be connected to your iPad with the Apple adapter appropriate for your model of iPad - if the HDD is both partitioned and formatted appropriately (i.e., a single partition - formatted as FAT32, exFAT, APFS or HFS+). 

You should also note that the iPad is not capable of providing sufficient power to run an HDD. If the HDD does not have its own source of power - then external power must be supplied to the HDD using a connecting adapter (e.g., an Apple Lightning to USB3 Camera Adapter and Apple Charger of at least 12W) - or by connecting both the iPad and HDD to a powered USB hub.

If you iPad has Lightning Port, you will require a Lightning to USB 3 Camera Adapter:

https://store.apple.com/uk/xc/product/MK0W2ZM/A

I hope you find this to be helpful in providing some useful clarity.

Hi! Do you mean an iPad 1st gen from 2010? Sorry but if it's not through lightning and the latest iPadOS this won't be possible.

You are very welcome.

To clarify, the Power Adapter must be connected to the pass-through Lighting connector of the Lightning to USB3 Camera Adapter. The Power Adapter will provide power to run the HDD - and simultaneously provide power for the iPad.