Omen of Pestilence can be used to block incoming skillshots.
Omen of Death can become more reliable if you cast it right before allies die, but it is less powerful.
During Omen of Death, both the revenant and reanimated champion count as a summon toward Unholy Covenant's bonus.
You can use Summoner Spell Smite on Yorick's minions to immediately negate their effects.
When Yorick uses his ultimate on an ally, it may be easier to kill the revenant before focusing back on the target who would otherwise revive.
The amount of health restored by Omen of Famine is based on the actual damage dealt, so Magic Resistance will lower its effectiveness.