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, not: A057104
octal numbers: A007094
octupi: A029767 *
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
operational recurrences: A001577 *
optimal rulers: see perfect rulers
OR(x,y): A003986 *
OR: A007460 , A006583
orchard problem: A003035 *, A006065 , A008997
ord(x,y): the multiplicative order of x mod y, see entries under: multiplicative order
order, binary: A029837
order, multiplicative order of 2 mod n: A002326
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, number of: A039931 *, A039927 *, A048885 *
orthogonal arrays, see also: A008286 , A039930 , A048164 , A048638 , A048893 , A049082 , A049083
orthogonal groups: A003053 *, A001051
out-points: A003025 , A003026