that is logically correct, or you can processes all the do somethings together.

if (mid(OpRx_$,1,4) = B))
{
do something1;
}
else if (mid(OpRx_$,1,5) = C))
{
do something2;
}

else if(mid(OpRx_$,1,4) = E))
{
do something3;
}
else if (mid(OpRx_$,1,5) = F))
{
do something4;
}

when ever OpRx_$ changes , a new thread will start processing, and it will do the appropriate something.
you might want to prevent reentry if that is a problem for your do something functions. i cant say without knowing what they do.