search string in varchar field

Hi,

I  get the ORA- 01722 error (invalid number string) when I try to convert
varchar2 to number using to_number function.
Case 1:
I have all numbers in varchar2 but it seems there is some character value
too. Is there a way that I can know that field is. The table is huge
(>13,000,000 rows)

Case 2:
When I query "select to_number(oci_id) from oci_tb; ", I don't get any
error.
But I get ORA - 01722 when I query "select oci_id from oci_tb where
to_number(oci_id) > 10000; ". 
I have no clue why I get error when where cluse is used.

Any suggestion?

Thanks
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