Yesterday I saw . . . . . . . . owl outside the window.

An indefinite article is used when we are talking generally about something.
The indefinite articles are 'a' and 'an'. 'An' is used before a word starting with a vowel or vowel sound. 'A' is used before a word starting with a consonant or consonant sound.
As 'owl' is starting with a vowel, the blank will take 'an'.