It’s this meaning:
[int] oh; ho; exclamation of surprise, admiration, etc.
Correct. It’s for writing.
Edit: Forgot to mention, the かける is 書ける.
Edit 2: That goes for both かける. Knowing it’s the potential for write (“can write”), this should make more sense:
「これはじがかけるいし！すごいよくかける！しろく」
Edit 3: I just noticed, I missed the last @_Marcus post above mine here. Oops.
The よく here will be “well”.