Question

Number of groups or atoms which have higher priority than $-OC{H}_3$ according to CIP rule.

Answer 1

$-\stackrel{18}{OH}-\stackrel{14}{-OC{H}_3},-SH,-O-CH=C{H}_2,-F$

$-\stackrel{18}{OH}$ will have higher priority over $-OC{H}_3$ because it has higher isotopic mass of $O$.
$-\stackrel{14}{-OC{H}_3}$ will have higher priority over $-OC{H}_3$ because it has higher isotopic mass of $C$.
$-O-CH=C{H}_2,$ has higher priority because it is equivalent to having two carbons attached to $O$.