Its also being used in the Standard format, not only as a control finisher, but also as a way to mill an opponent after Doomsday Excruciator leaves them with just a few cards left in their libraries
For this reason, the card is most at home in an Aristocrats-style deck
O-Kagachi is in this weird spot where there's much better beaters at 6 mana (heck, Territorial Hellkite is a better beater at 4) and there's much better rattlesnakes at much less mana, such as Karmic Guide, Soul Snare, even Seal of Primordium
Ulamog then gains annihilator X, with X equalling the number of +1/+1 counters on it, making it a devastating ability
Sandman's first ability works in all zones, not only the battlefield
You can also open cards from the Streets of New Capenna Commander set in extended art in Collector Boosters