Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. oppressus (obp-), a, um, Part., from opprimo.
2. oppressus (obp-), ūs, m. [opprimo], a pressing down, pressure (poet. and in post-class. prose): in oppressu valido, Lucr. 1, 851: montis, Sid. Ep. 9, 11.