Reach and AoO's

Something dreadful happened to 5e monsters: their arms got shorter - a lot shorter. Many large creatures (e.g. ogres, trolls, golems) had their reach shortened when WotC decided to nerf Attacks of Opportunity in the name of combat simplicity and to allow combat to be a lot more 'dynamic'. To address this sad state, the following now applies:

• All such monsters (nominally Large or larger monsters, especially those with melee weapons but also those with long necks or tentacles) now have at least a 10ft reach
• A character with a 5ft reach may cause an Attack of Opportunity if they attempt to close with such a creature
• This AoO follows all the normal rules for an AoO (notably, it still uses the creature's Reaction to make the AoO)