According to ETNews the Galaxy Note 4

was originally going to have a 20MP sensor but Samsung dropped it to 16MP (the report actually says 12MP, but we now know that's wrong) to keep it thin while still including OIS, however the company will instead use the 20MP sensor on the Galaxy S6 apparently.

What it doesn't say is whether the S6 therefore won't include OIS or won't be all that slim, since reading between the lines one or the other is likely to be the case. In fact at 8.5mm thick the Note 4 is already slightly fatter than the Galaxy S5, so if the Galaxy S6 does get OIS then it's likely to put some weight on too.