Onsight does not send any order numbers or invoice numbers to MYOB. The order numbers and invoice numbers are auto-generated by MYOB from the sales numbering settings that you have set up.

This is where you can find your settings for the web:

This is where you can find your settings for desktop:

Here are the MYOB support docs for how to manage these settings:

https://www.myob.com/au/support/myob-business/sales/choose-how-to-number-sales-transactions