Nabab Will

Napier's Bones

Napier's Chessboard

Napoléon Triangle

Narcissistic Chain

Narcissistic Number

Nasik Square

Naughts and Crosses (1)

Naughts and Crosses (2)

Necker Cube

Needles

Network

Nine

Nine Dots

Newton's Cows

Newton's Trees

Nichols Cube

Nicomaque Triangle

Nim

Nine Men's Morris

Niven Number

Nodal Point

Nolid

Number

Nomogram

Non-Homogeneous Tessellation

Nonagon

Nonagonal Number

Normal Form

Normal Magic Lattice

Normal Magic Square 

Normalized Latin Square

Normalized Latin Rectangle

Notched Square

Numeration

Numerical Buckle

Numerical Chain

Numerical Curiosity

Numerical Tessellation

Numerical Pyramid

Numerical Recreation

Numerical Tic-Tac-Toe

Numerical Transformation

Object

Oblique Row

Oblong Number

Octabolo

Octacube

Octadigital Identity

Octagon

Octagonal Centered Number

Octagonal Gnomonic Number

Octagonal Hyperpyramidal Number

Octagonal Number

Octagonal Pyramidal Number 

Octagrade

Octagram

Octahedral Die

Octahedral Number

Octahedron

Octahex

Octahexaflexagon

Octiamond

Octofoil

Octomino

Octomorphic Number

Octopawn

Octoperfect Number

Octosceles

Odd Number

Odd Vertex

One-Dimensional Jigsaw

One-Dimensional Maze

Optical Illusion

Order

Ore Formula

Organigram

Origami

Orthodox Chess

Orthogonal Latin Square

Orthogonal Line

Orthogonal Symmetry

Othello

Overhand Knot

Ozanam Cards

Ozanam Dinners