Unless an effect allows you to play additional lands that turn, you can play a land card exiled with Ob Nixilis only if you haven't played a land yet that turn.

If you don't play the card, it will remain exiled.

You may play the card exiled with Ob Nixilis even if Ob Nixilis leaves the battlefield or you lose control of it.

Playing the card exiled with Ob Nixilis follows the normal rules for playing that card. You must pay its costs, and you must follow all applicable timing rules. For example, if the card is a creature card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.

Ob Nixilis's last ability looks at all combat damage dealt to an opponent in total to determine whether it triggers. For example, if three 1/1 creatures deal combat damage to an opponent, that player loses 3 life. Ob Nixilis's ability won't trigger.

Ob Nixilis's last ability will trigger if an opponent is dealt exactly 1 damage, pays exactly 1 life, or loses exactly 1 life some other way.