I thought the words were categorized by masculine and feminine using the letters "o" and "a"
That's just a general rule. You have to memorize them. For example, the hand is la mano, so a hand would be una mano.
In general, words ending in 'a' are feminine, and 'o' are masculine, but there are few exceptions such as 'mano' (feminine)
Why is a street feminine? And a bus masculine? And a suitcase feminine? I was thinking it was because calle ends as a vowel and autobus ends as a consonant
I typed "una ca" and as I was reaching the L on my keyboard my finger slipped and pressed enter.