In most contexts, on [doing something] and after [doing it] are equivalent and interchangeable. But there can be a nuance of difference, in that strictly speaking the on version implies when, at the actual time of [doing it], whereas after obviously implies a (usually only very slightly) later time.
It's tricky to find an example context that's likely to clearly distinguish between the two, but consider...
1: On taking off his shirt he found there was a button missing
2: After taking off his shirt he found there was a button missing
Perhaps it's just me, but I find #1 far more natural, since the fact of the missing button would become apparent during the shirt removal process, not after it.
Okay - here's another one...
3: On entering a new hotel I always tip the doorman
4: After entering a new hotel I always tip the doorman
...where again the first version makes more sense to me, since I picture typical hotel doormen as standing outside the front entrance. I'd like to picture myself as a wealthy patron casually slipping the doorman a substantial tip as he doffs his cap / tugs his forelock and holds the door open for me to go through, thereby ensuring that I'll get attentive service from him for the duration of my stay.
EDIT: I should point out that the preposition on in my examples #1 & #3, and "equivalent" alternatives such as when, while, during,... are syntactically optional, and don't affect meaning.
By default, the relationship between the primary clause (I tip the doorman) and the participial clause (entering a hotel) is one of simultaneity. But other "time-based" prepositions (before, after, since,...) indicate different temporal relationships, which DO affect meaning.