Is it possible to add comments to an OE once it has been invoiced and posted? My GM would like to be able to "note" things when problems come up, like service notes, but it does not seem like you can edit an OE that's in history and can only view it.

You could add notes to the invoice on the customers card instead. With the AR card in receivables view, highlight an invoice and click the details button. There is a notes button on the details screen where you can add comments (and timestamp if needed). When notes have been added to an entry, the notes indicator will contain a check mark when viewing the customers card.