To be sure O/S is explained correctly, it is the number of shares that have been issued of the total number of authorized. To use the credit card analogy does not quite work because many of those shares are owned by insiders, thus not credit.
I believe you mean float, when you speak of it in terms of credit, which is Outstanding Shares (O/S) held by the public, other than insiders, which would be more indicative of "credit" as you made the analogous reference to.
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