When "LOI" is used, it actually refers to the Letter of Indemnity. This is standard usage of the term and has been for decades. The Letter of Intent (hereafter referred to as LOI) is often used in specific domestic trades, such as in real estate or corporate mergers, but it has no standard usage in International Trade itself.
Finally, there is no such thing as an oral offer! An oral offer is really just a test by the buyer to see what the seller will or will not accept. Make the buyer put the terms of the offer in writing as an intermediate step toward securing a legitimate offer.