that's not the convention I know... (from CLRS algorithms book).
Big O is sort of like <=. little o is sort of like <.
Big O is sort of like <=. little o is sort of like <.
O(n) is: "there exists a K such that, for sufficiently large n, K*n is always larger".
o(n) is: "for every K, for sufficiently large n, K*n is always larger". (So o(2n+1) and o(2n) and o(n) are the same thing.)
For what it's worth you can leave out the 'sufficiently large n' part, assuming we're talking about positive integers (or continuous functions on the nonnegative reals). I like to leave it in though because it makes the jump to defining both o, O and Theta in terms of lim sup f/g a little easier.