I do not know with OSA, but I suppose it will be the same as with other SAM systems when you turn missiles on and in the mean time you do other things (no target can be acquired at all).

If the preparation was part of the firing sequence, you would lose 12s in the case of OSA and given the fact it is a short range system, you could be dead or the target could be away (not even talking about longer times with other SAMs).