oi provides a location-annotated error type so you can display useful error messages to your users.
Error messages without location information are often not actionable, especially when they come from a complex program with many potential sources of errors.
Compare an unannotated error:
sh
$ foo
No such file or directory (os error 2)
…with an annotated error:
sh
$ foo
Configuration.toml: No such file or directory (os error 2)
oi is named after the exclamation, as in Oi! Oi! Oi!. Imagine alerting your users to the location of errors: "Oi! 1.2.3.4 is unreachable!"