@dlowan,
That's terribly sad, Deb. Poor Oscar.
I saw a similar reaction in one of my moggies a few years ago, when her beloved feline partner in crime died. She just sat, motionless, on the couch in the loungeroom, refusing to play, go outside or even move around much. This lasted for a couple of weeks.
In the end we had to take matters in our own hands - gently pushing her outside (so she had to use the window or cat door to return inside - then we'd gently push her out again ...). And lots of playing with her, throwing balls for her to retrieve, etc .. heaps of attention. Poor Mozz, she took that death
very hard.