I sold stock under an ESPP and I received a supplemental cost basis on my 1099-B for income that was already reported on my W2. How do I change the reason code in F to O

I believe the default code entered by TurboTax for corrections to basis - I assume you used the ESPP "guided" interview here - and the explanation for that code is 'You received a Form 1099-B (or substitute statement) and the basis shown in box 1e is incorrect..." and that's exactly right for your situation.  There's no code for "this was ESPP stock and the broker is only showing my out of pocket cost and not including the compensation created by the sale."  In any case, reporting your income correctly is really your only absolute obligation here, as a citizen.  The IRS just can't hassle you and won't hassle you over "style" issues, which is what these codes amount to.

The default code is Code O.   And as stated above, this is the correct code for your situation.   You can view Form 8949 to see the adjustment code for column(f) for the adjustment amount made in column(g).