Welcome! to Duolingo, Welsh. :)
Is it used the same as in English? I mean, both for greeting (e.g. upon arrival) and as a reply for thanks? (In my language these two are completely different.)
Yes you can use it for both like English.
How's the o at the end supposed to be pronounced?
like the 'o' in 'robOt' or 'cOt'. Don't pronounce it like the long 'o' in 'bOAt' or 'thOUGH'.
Like the au in caught or the ou in bought or brought.