How can you distinguish between 'とう' and 'とお'?

[ItsTheSebbe]

I find myself sometimes confusing the ‘とう’ and ‘とお’. For example father, お父さん is transcribed as おとうさん, whilst tenth day, 十日 is transcribed as とおか.

I often accidentially input とうか for tenth day, which is counted as incorrect. I can’t hear any differences in pronounciation between the ‘とう’ and ‘とお’. So I wonder how you can distinguish the two and predict which of the ones you will have to use.
Thanks!

[MissMisc]

It’s a difference in spelling, same sound though (long お)

おう is usually in on’yomi readings, and おお is usually in kun’yomi – etymology-wise, words seen with the おお spelling used to be おほ or おを, and have since become the おお we see today (other examples: 通 とおり , かき氷 ごおり, etc)

[banira]

Also, most of the time it’s とう. There are only a couple of words that use とお, so you should probably consider them an exception and learn them separately.