This might sound really dumb and obvious, but have you tried feeding him flakes? I had this problem with my new baby solver arowana. The lfs said they fed him crickets and pellets, but when I got him home, he didn't touch either of them. So I tried to starving him out until he took the pellets and he went 4 days with no food so I got worried. Then out of curiousity I threw some flakes in there and he just inhaled them. So, it might be that your oscar is too small to recognize anyhting other than flakes. I really doubt lfss feed their tiny fish anything other than flakes so give them a try.
