This is the 202nd most frequent Czech word.
jednoho
"Jednoho" means "of one" or "one" (genitive or accusative singular of "jeden" - masculine).
In this sentence, 'jednom' is in the locative case to agree with the preposition 'o' which governs the locative when discussing a topic or entity.
Mluvili jsme o jednom zajímavém projektu.
We talked about an interesting project.
The word 'jednou' is in the instrumental case, used to indicate the means or accompaniment of the action of meeting.
Potkal jsem se s jednou starou přítelkyní.
I met with an old female friend.
Here, 'jednoho' functions as a genitive singular form of the word 'jeden' (one), used as an attributive adjective to describe an indefinite day.