Any time a Onewheel is powered on or off, the board goes through a system check, part of that includes the Lightbar. The Lightbar will briefly flash green on the right side when it is checked and will turn off. This blink does not constitute an error.
If your board does experience an error, the Lightbar would flash yellow or red a number of times related to the error. If you are experiencing an error, please read this section on decoding your Onewheel to get the error code and determine if you need further support!