Macronized Final Vowel Rule for O and U

Simply put, the final -o and -u are always -ō and -ū. These forms are required when suffixes are added, rendering the O or U as no longer final.

Lo ---> Lōsu (That ---> Those) /lo:/ ---> /'lo:su/
Tu ---> Tūsu (Heart ---> Hearts) ---> /tu:/ ---> /'tu:su/