Yes. I take lids off to take samples, dry hop, smell it, etc, all the time. In 13 years of brewing I've never oxidized a beer. I don't take the lid off for long, and I don't swirl it around or move it with the lid off, but yes, you can, as the co2 covering the beer will take a while to dissipate if you take care not to cause any turbulence in the fermenter by moving it or causing the beer to slosh around.