This is just a question after I keep making the same mistakes during my reviews. As I understand, the お- or う- character is sometimes placed after an o-vowel to make a long vowel.
とおか - Too-ka - Ten Days
ようか - Yoo-ka - Eight days.
I often make the mistake by writing a word with お where it should have been う and vice versa. So I was wondering if there was some system to all this, which could help me in my learning.