hi
@dawdle004, welcome to the wonderful and sometimes confusing world of Japanese!
As you've already noticed, some words have long vowels (called 長音, chōon, "long sound"), which I prefer to represent in rōmaji with a macron (ā, ī, ū, ē, ō) instead of spelling out the vowel twice (how it would sound phonetically), or as it would be written in hiragana (this is a common approach, as Buntaro wrote "koujiki", for example), which can confuse English speakers into thinking it's a dipthong (where two distinct vowel sounds are paired, like "ew" or "aye").
The fundamental reason for this distinction, as you'll learn, is that Japanese is a moraic language, which means that the fundamental phonological unit of the language, called a mora, which is represented by individual kana characters (with some exceptions but we'll keep it simple).
It's best to think of kana like notes on a sheet of music, and each note has the same time signature. A long vowel, then, is a note extended by one count. This is why Japanese people will sometimes clap as they "sound out" a word, indicating how many mora are in the word.
As Buntaro mentioned, a standard vowel sound and a long vowel are distinct from each other in Japanese, meaning you can create "minimal pairs," which is a fancy way to say two words that are exactly the same except for one sound (e.g. cat and hat, peg and pig, willow and pillow). The difference between a vowel and a long vowel is as obvious in Japanese as these examples in English. So in Japanese, "osama" and "ōsama" are literally as different as "Osama (bin Laden)" and "king".
now, WHY they are written differently in hiragana is a good question, and I'm sure there's some convoluted historical reason for this quirk...
A little internet search later, YEP!
What is historical kana usage?
As with many things in Japanese, you have the option to just learn the basic rules and get used to it, or to do a deep dive into the reasoning behind it, which (
WARNING:) may be as enlightening as it can be confusing for a beginning language learner.
To start, definitely work on getting hiragana down, understand that there are long vowels (and long consonants as well, but that's a different conversation), and that they are crucial for properly understanding and pronouncing Japanese vocabulary. You don't need to fully understand the "why" in order to use it properly, but it does come in handy when you want to write a lengthy forum post.