The OCS OverRun rule explained

Some new players, including myself, were confused about how overrun works and what hexes you can or cannot do an overrun in. This was not very clear in the rulebook. However if you think of it in terms of the ATTACKING HEX, NOT ONLY THE DEFENDERS it starts to get clearer. I read in a PDF document example of play on the OCS Depot about an overrun example and this cleared it up for me

Preparatory Moves:
I’d like to overrun along the road from 17.27 to 18.27, unfortunately rule 8.1c forbids the use of road
for overruns, as your units are actually maneuvering in the hex to outflank the defender. Additionally
rule 6.1d says you can move from hex A to B only if you could also move from hex B to A, this is
logical though a bit unusual. In the example below units using track or truck type MP cannot move
from 18.27 to 17.27 without the road as they are prohibited to enter swamp. So, in the other direction,
from 17.27 they cannot leave the road to overrun 18.27. All in all you cannot overrun from a swamp
hex with units using track or truck MP, although you could with leg MP type units.

This document is located at:

http://www.ocsdepot.com/images/contentfile/Smolensk_Northern_Pincer_AAR_v12.pdf

Example is on page 12.

Furthermore I got a clear explanation on the OCS Facebook page as well. It was as follows in summary:

 9.1f A unit can never attack a hex that the movement rules (see especially 6.1d and 6.2c) prohibit it from entering. This applies even if the TEC seems to indicate otherwise. Note also that if a hex can be entered in movement, but only by using a road, a regular combat is allowed, but not an overrun (8.1c).

To explain a bit better: the first thing to do when working out if an overrun is possible is ignore any roads/bridges (8.1c). That means no overruns into a hex which has a non-road cost of 4 or more.

The next thing is to remember 6.1d (while still ignoring roads): you can only move from A to B if you could also move from B to A. If terrain with a movement cost of "P" is adjacent to an open hex, that means no overruns in or out of the "P" hex. 

 

No ordinary movement either - but a road will make ordinary movement possible.