The ora was an ounce. Originally a Danish unit, it was normally worth 16 pence. However, when making allowance for depreciated coins, the ora was assumed to be 20 pence; that is, for every 16 pence due, 20 pence in coins was collected.
For more detail on the Domesday ora, see Sally P.J. Harvey, 'Royal revenue and Domesday terminology', Economic History Review, second series, vol. 20 (1967), pages 221-28; and Sally P.J. Harvey, Domesday: Book of Judgement (2013), chapter 6 and appendix 1..
See also assayed.