The basic principle of BOP accounting is double entry bookkeeping. In short, every transaction enters twice. As a consequence of this, by definition
$$\text{Current account } (CA) + \text{Capital account } (KA) = \text{Financial account } (FA)$$
Note that this may differ depending on how you classify BOP accounts. Some sources classify it as CA = KA (and do not have an FA), but the principle is the same.
In my definition, the FA includes the official reserve account (OA).
What I don't understand is - if every transaction automatically generates a credit and debit entry, how can it be possible to have:
$$CA + KA \neq (FA \; \backslash \; OA)$$
I know I am wrong to think this, but it seems to me like the 'need' for central bank intervention to absorb a surplus/deficit in the other components violates the definition of double-entry bookkeeping, which implies BOP = 0 at any point in time. So:
1) Why am I wrong wrt the above?
2) If the OA does not absorb the deficit/surplus from the rest of the BOP, what happens?