Seeing a black cat cross your path is considered a bad omen.
good omen
Frequency:
75
%
Finding a four-leaf clover is believed to be a good omen.
ominous presence
Frequency:
50
%
The figure standing at the edge of the graveyard had an ominous presence.
Idiomatic Expressions for omen
Phrase: Omen of things to come
Meaning: A sign or indication that suggests what will happen in the future.
Usage: The sudden drop in the stock market was seen as an omen of things to come for the economy.