Im am trying to calculate the following. What is the best way to do it.

if C2 < C1 (value of prev row same col)
and if D2="O" or "B" then F2= 1 (numeric result)

else if C2 = C1 (value of prev row same col)
and if D2="B" then F2= 2 (numeric result)
or if D2="S" then F2= 3 (numeric result)
or if D2="X" then F2= 4 (numeric result)
else "Error"

How would I write the formula??