An Over the Air (OTA) SMS is an SMS message that is frequently used by mobile operators to provision or modify core network settings on a handset. These messages are used to optimize configuration settings e.g. SMS Service Center, web settings, or to provision new services such as MMS.
Over the Top (OTT) messages are messages that flow through the data network of a mobile operator as opposed to the core network. OTT suggests something that flows ‘over the top’ of the core network. For example, a P2P SMS message travels through the core signalling infrastructure of a mobile operator, where a WhatsApp message will flow through the data connection of a mobile operator (just like any other data). In this context a WhatsApp message is regarded as an OTT message.