Is eating one whole olive considered as a kezayit (for the purposes of a bracha acharona)? Or is the pit, which most people do not eat, not considered - making only the remainder of the olive (which of course is less than a kezayit) count towards the shiur?
The Talmud explicitly addresses this.
אמר ליה רבי ירמיה לרבי זירא רבי יוחנן היכי מברך על זית מליח כיון דשקילא לגרעיניה בצר בצר ליה שיעורא אמר ליה מי סברת כזית גדול בעינן כזית בינוני בעינן (והא איכא) וההוא דאייתו לקמיה דרבי יוחנן זית גדול הוה דאע"ג דשקלוה לגרעינותיה פש ליה שיעורא דתנן זית שאמרו לא קטן ולא גדול אלא בינוני וזהו אגורי ואמר רבי אבהו לא אגורי שמו אלא אברוטי שמו ואמרי לה סמרוסי שמו ולמה נקרא שמו אגורי ששמנו אגור בתוכו
R. Jeremiah asked R. Zera: How could R. Johanan make a blessing over a salted olive? Since the stone had been removed, it was less than the minimum size! — He replied: Do you think the size we require is that of a large olive? We require only that of a medium sized olive, and that was there, for the one they set before R. Johanan was a large one, so that even when its stone had been removed it was still of the requisite size. For so we have learnt: The ‘olive’ spoken of means neither a small nor a large one, but a medium one. This is the kind which is called aguri. R. Abbahu, however, said: Its name is not aguri but abruti, or, according to others, samrusi. And why is it called aguri? Because its oil is collected [agur] within it. (Soncino translation)
In other words the size of a kezayis is the size of a medium olive including the pit. Therefore, eating a medium olive without the pit would not be considered eating a kezayis, but eating a large olive without the pit would be considered eating a kezayis.
If we understand a kezayit to be the volume of an olive (rather than a complex measure per R' Chaim Naeh et al.), then a single olive is not a kezayit, as the pit is considered within this volume.
In this case, a kezayit would be about one and one half olives, excluding their pits.
As always, CYLOR