Use nested IIFs or a translation table.
New_Field_Name: IIF([YourField] = "O", "Other", IIF([YourField] = "B",
"Bill To", IIF([YourField] = "P", "Prepaid", "Unknown")))
Translation_Table --
Letter - text
Word - text
Letter Word
O Other
B Bill To
P Prepaid
Then in your query left join [YourField] to [Translation_Table].[Letter]
field and add [Word] as an alias field.