Even though most probably a Middle English loanword, the compound oar-piece does not seem to be attested in the English dictionaries.

An alternative interpretation of this single citation would be that the word has no relation to the naval context, but that John of Gaunt simply orders to grant thirty-six 'pieces of gold' to the two citizens of Norwich in addition to the ‘two masts’ (cf. the locution piece d’or sub or¹). However, a more precise (historical) context is lacking.