Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ob-tentĭo, ōnis, f. [id. II.], a covering, veiling; trop., obscurity, darkness (postclass.): aliquid obtentionibus allegoricis claudere, by an allegorical dress, Arn. 5, 181; 5, 182.