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# Index to OEIS: Section O

# Index to OEIS: Section O

- This is a section of the Index to the
**OEIS®**. - For further information see the main
**Index to OEIS**page. - Please read Index: Instructions For Updating Index to OEIS before making changes to this page.
- If you did not find what you were looking for in this Index, you can always search the database for a particular word or phrase.
- Full list of sections:

[ Aa | Ab | Al | Am | Ap | Ar | Ba | Be | Bi | Bl | Bo | Br | Ca | Ce | Ch | Cl | Coa | Coi | Com | Con | Cor | Cu | Cy | Da | De | Di | Do | Ea | Ed | El | Eu | Fa | Fe | Fi | Fo | Fu | Ga | Ge | Go | Gra | Gre | Ha | He | Ho | Ia | In | J | K | La | Lc | Li | Lo | Lu | M | Mag | Map | Mat | Me | Mo | Mu | N | Na | Ne | Ni | No | Nu | O | Pac | Par | Pas | Pea | Per | Ph | Poi | Pol | Pos | Pow | Pra | Pri | Pro | Ps | Qua | Que | Ra | Rea | Rel | Res | Ro | Ru | Sa | Se | Si | Sk | So | Sp | Sq | St | Su | Sw | Ta | Te | Th | To | Tra | Tri | Tu | U | V | Wa | We | Wi | X | Y | Z | 1 | 2 | 3 | 4 ]

O'Nan group: A003919, A008625

obtaining numbers from other numbers and the operations of addition, subtraction, etc: see under four 4's problem

octagonal numbers: A000567*

octahedral numbers: A005900*

octahedron, truncated: see truncated octahedron

octahedron: A005899

** octal numbers, sequences related to : **

octupi: A029767*

** odd numbers , sequences related to : **

- odd numbers n such that 2^k + n is composite for all k: see A076336
- odd numbers, fake: A080591
- odd numbers: A005408*
- odd numbers: see also A000700, A000069, A007697, A006046, A007455, A007482, A000593, A007483, A006945, A001033, A002309, A006285, A002594, A006038

odd unimodular lattices, see: lattices, unimodular

odious numbers: A000069*

omega(n), number of distinct primes dividing n: A001221

Omega(n), total number of primes dividing n: A001222

one local maximum, arrays with: A007846, A000079, A087518, A087783*, A087923-A087932

one odd, two even, etc.: A001614

one puddle: see one local maximum

ones-counting sequence: A000120

open problems: see also unsolved problems in number theory (selected)

open problems: try searching in the OEIS for the following words: conjecture, apparently, appears, seems, probably, etc.

operational recurrences: A001577*

Opmanis's nice base-dependent sequence: A177834

optimal rulers: see perfect rulers

OR(x,y): A003986*

OR: A007460, A006583

orchard problem: A003035*, A006065, A008997

** order or orders, sequences related to : **

- order, binary: A029837
- order, multiplicative order of 2 mod n: A002326
- order, ord(x,y): the multiplicative order of x mod y, see entries under: multiplicative order
- order: see also under multiplicative order
- ordered factorizations: A074206*, A002033
- ordered partitions: see also under partitions
- orders, total: see total orders
- orders, weak: A000790
- orders: A000670, A004123, A004122, A004121
- orders: see also hierarchies

ordinals: A005348

Ore numbers: A001599*, A001600

** orthogonal arrays, sequences related to : **

- orthogonal arrays, number of: A039931*, A039927*, A048885*
- orthogonal arrays, see also: A008286, A039930, A048164, A048638, A048893, A049082, A049083

orthogonal groups: A003053*, A001051

out-points: A003025, A003026

overpartitions: A015128

- This is a section of the Index to the
**OEIS®**. - For further information see the main
**Index to OEIS**page. - Please read Index: Instructions For Updating Index to OEIS before making changes to this page.
- If you did not find what you were looking for in this Index, you can always search the database for a particular word or phrase.
- Full list of sections:

[ Aa | Ab | Al | Am | Ap | Ar | Ba | Be | Bi | Bl | Bo | Br | Ca | Ce | Ch | Cl | Coa | Coi | Com | Con | Cor | Cu | Cy | Da | De | Di | Do | Ea | Ed | El | Eu | Fa | Fe | Fi | Fo | Fu | Ga | Ge | Go | Gra | Gre | Ha | He | Ho | Ia | In | J | K | La | Lc | Li | Lo | Lu | M | Mag | Map | Mat | Me | Mo | Mu | N | Na | Ne | Ni | No | Nu | O | Pac | Par | Pas | Pea | Per | Ph | Poi | Pol | Pos | Pow | Pra | Pri | Pro | Ps | Qua | Que | Ra | Rea | Rel | Res | Ro | Ru | Sa | Se | Si | Sk | So | Sp | Sq | St | Su | Sw | Ta | Te | Th | To | Tra | Tri | Tu | U | V | Wa | We | Wi | X | Y | Z | 1 | 2 | 3 | 4 ]