Note 5 already has small battery. Always on display will kill the battery faster. And note 5 is on older software, not latest Samsung experience. I think it's on grace ui.
1st off the Note 5 will not get the Android Oreo Update. A major revamp of software is extremely unlikely. Moreover the Note 5 doesn't use the proper screen to support AOD. It does have an AMOLED Screen but it's not capable for AOD. Moreover there are apps on the play store which enable AOD but I would refrain from using them because they could shorten the life of your screen.
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