Learn Polish: Znany reżyser ma już trzy Oscary. - The famous director already has three Oscars.

Both are seen in Polish, but dictionaries recommend keeping the original English spelling with the letter c for the Academy Award. Because it ends in a hard consonant, we just add the standard plural -y directly to the foreign stem to get Oscary.

Since the name Oscar is masculine personal, why is the plural Oscary instead of Oscarzy?

When Oscar refers to the statuette or the prize, it loses its personhood and becomes a masculine inanimate noun. Therefore, it takes the standard masculine inanimate plural ending -y. If you were talking about actual men named Oscar, the plural would indeed be Oscarzy.

Why do we say trzy Oscary instead of trzech Oscarów?

Because the award is treated as an object, it pairs with the basic number trzy, which is used for inanimate objects, animals, and feminine or neuter nouns. You would only use trzech if you were counting masculine personal nouns, which would force the noun into the genitive case.

Reżyser is also a loanword. How does it decline in the plural?