A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

one-on-one • one-to-one
one-on-one —(attrib.) 
1
. involving a direct contact between
one person and another: Our school offers both one-on-one
instruction with your own teacher and group instruction. 2.
playing or fighting directly against a single opponent: I was
quickly engaged in one-on-one combat with a tall, lean war-
rior apart from the central conflict.
one-to-one —
1
. said of pairing each element of a set with an
element of another set: I strongly identified with this philos-
ophy of enhancing social skills in children by pairing them
one-to-one with college students. 2. = one-on-one 2: The brawl
began as a one-to-one fight between a black inmate and a
Latino, and escalated into a small melee with up to a dozen
inmates.
Note: 
Neither expression correlates in meaning with the phrase
one by one
— one after another: I found these group ward
rounds more efficient and also more enjoyable than seeing
patients one by one.
one step ahead • one step further
one step ahead —a small advantage in a competitive situation:
Credit card companies are turning to so-called smart cards in
an effort to keep at least one step ahead of the scammers.
one step further —at a more advanced level: If we can transfer
this knowledge to the society we are one step further and the
World Science Forum reaches one of its goals.
one way or another • one way or the other
one way or another —
1
. (also: in one way or another) by some
means or other: They thought always about winning, and,
one way or another, they almost always did win. 2. for any of
various reasons: Captain Fitzgibbon wouldn’t come back from
that last Ulster tour, one way or another.
one way or the other —regarding either of the two possibil-
ities: These men cannot both be correct so would somebody
produce some evidence to prove the point one way or the
other?