Oathbreaking (n.)
definition: The violation of an oath; perjury.

Oathbreaking is a 12 letter word that begins with an O and ends in a G

Obcompressed (a.)
definition: Compressed or flattened antero-posteriorly
Or in a way opposite to the usual one.
Obcompressed is a 12 letter word that begins with an O and ends in a D

Obedienciary (n.)
definition: One yielding obedience.

Obedienciary is a 12 letter word that begins with an O and ends in a Y

Oblanceolate (a.)
definition: Lanceolate in the reversed order
That is narrowing toward the point of attachment more than toward the apex.
Oblanceolate is a 12 letter word that begins with an O and ends in an E

Obligatorily (ad)
definition: In an obligatory manner; by reason of obligation.

Obligatorily is a 12 letter word that begins with an O and ends in a Y

Obliterating (p.)
definition: of Obliterate

Obliterating is a 12 letter word that begins with an O and ends in a G

Obliteration (n.)
definition: The act of obliterating
Or the state of being obliterated; extinction.
Obliteration is a 12 letter word that begins with an O and ends in an N

Obliterative (a.)
definition: Tending or serving to obliterate.

Obliterative is a 12 letter word that begins with an O and ends in an E

Oblong-ovate (a.)
definition: Between oblong and ovate
But inclined to the latter.
Oblong-ovate is a 12 letter word that begins with an O and ends in an E

Obmutescence (n.)
definition: A becoming dumb; loss of speech.

Obmutescence is a 12 letter word that begins with an O and ends in an E

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