Are OTAA keys displayed in little or big endian format?
Strictly speaking, the keys are a byte-stream, there is no endian’ness. From a practical point of view, they should be entered as displayed when working with a module, or a byte-array in your code.
The LoRa-Alliance has documented best practices regarding the display of EUI’s and Keys, and Senet follows these (these also comply with the IEEE’s recommendations for EUI’s).
For some additional background, Endian’ness applies to an internal (byte or bit) representation of a “number”. The EUI is a 64bit number, thus the endian’ness of the representation in memory on the device and over the air needs to be specified. LoRaWAN is one of only two (that I know of) widespread networking protocols that encode numbers in little endian “on the wire” (the other is Zigbee). Your application is free to encode its payload data however you like. Senet encodes payload data in big-endian when using the Senet Packet Format.