In the visa forum, the acronym IO is variously used to mean "immigration office" or "immigration official"
In the context of this topic, which is about documents to be shown to the immigration official upon arrival at the airport, it can only mean "immigration official"
Every once in a while, I see a post recommending to line up at an immigration counter with a male official with the suggestion that female officials are more likely to ask probing questions or request documents like an accommodation reservation. I have seen no statistical evidence, though, to lend credence to this but it may well be the personal experience of the respective posters.