According to the Wikipedia definition of Dominant-Strategy Incentive Compatibility (DSIC):
DSIC means truth-telling is a weakly-dominant strategy, i.e. you fare best or at least not worse by being truthful, regardless of what the others do
I didn't find the formal definition of ex-post IC there, but I found this paper On the Foundations of Ex Post Incentive Compatible Mechanisms by Yamashita and Zhu. The ex-post IC is also defined as truth-telling is the best strategy no matter what the others do.
I am really confused about these two notations. I think there must be some difference between them, otherwise, it doesn't make sense to me why we develop two terminologies for one thing?