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(1) 1 is element of O, since 1 is an odd digit.
(2) 9 is element of O, since 9 is an odd digit.
The part "considering that 9 is not just divisible by 1 and itself?"
is set of words irrelevant to question 2.
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Comment from student: In what way is 1 an odd number? In what way is 9 an odd number?
My response: 1 is an odd number, because, when divided by 2 in integer numbers, it gives
the remainder of 1 (one). So, according to the definition of odd numbers, 1 is an odd number.
9 is an odd number, because, when divided by 2 in integer numbers, it gives
the remainder of 1 (one). So, according to the definition of odd numbers, 9 is an odd number.
Usually/normally, students learn these truths/conceptions in the second or third grade.
As I see from your posts, you try to review College Algebra without having necessary
elementary standard basic knowledge (prerequisites) related to the second and/or third grade.
If I remember correctly, I just made similar notice once before, responding to one of your preceding posts.
So, if you want to know my opinion, it is that you should not reviewing College Algebra,
but start from Arithmetic and then continue with Pre-Algebra.
It will better correspond to your current level in Math.