That sounds about right, and as Tom said, 30th on LI is pretty darn good.
Here's a list of 20+ courses that I believe are better than Glen Oaks:
Shinnecock, NGLA, Friars Head, Sebonack, Bethpage Black, Garden City, Maidstone, Atlantic, Piping Rock, The Creek, Deepdale, The Bridge, East Hampton, Meadow Brook, Southampton, Westhampton, St. George’s, Engineers, Nassau, Sands Point, North Shore, Bethpage Red, Rockaway Hunt, Huntington
Here are nearly 20 more, many of which are of similar (possibly better) quality, in my view:
Quogue Field Club, Brookville, Wheatley Hills, Mill River, Laurel Links, Montauk Downs, Hempstead G&CC, Woodmere, Rockville Links, Cherry Valley, Southward Ho, Bellport, Tallgrass, Bethpage Green, Cold Spring, Eisenhower Red, Garden City CC