Tom saw a person with a scary looking face today.
おっかない is a common colloquial expression. おっかない顔 means “a scary looking face” like Yakuza/mafia or “a furious looking face” like an angry mom, and it sounds very natural. However, the sentence had better be replaced with 見た instead of 見ました (formal/polite tone) for the better collocation with おっかない. I won’t say おっかない顔 + 見ました is grammatically incorrect or completely unnatural, though,