Your life total becomes equal to your starting life total. Lands you control don't untap during your next untap step.
7/14/2017 For your life total to become your starting life total (normally 20), you gain or lose the appropriate amount of life. For example, if your life total is 4 when Oketra’s Last Mercy resolves, it will cause you to gain 16 life; alternatively, if your life total is 25 when it resolves, it will cause you to lose 5 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.
7/14/2017 In a Two-Headed Giant game, Oketra’s Last Mercy causes the team’s life total to become the team’s starting life total (normally 30), but only you actually gain or lose life.
7/14/2017 No lands that you control will untap during your next untap step, even lands that aren’t tapped as this spell resolves. This includes lands that enter the battlefield after this spell resolves.
7/14/2017 If more than one spell says that lands you control don’t untap during your next untap step, the effects will all wear off during that untap step. You’ll untap lands you control during your untap step after that one.