"olen työssä" vs "olen töissä"
Could someone explain what the difference is please?
"Olen töissä" = "I am at work"
"Olen työssä" I have never heard anyone say this, but I googled it. Apparently some people use this, but it definitely isn't a common way of saying that you are at work. So I would use "Olen töissä"
The difference between these sentences is that one of them is singular and the other one is plural.
töissä=in work, in jobs
As a nuance note I understand olen töissä mostly as "I am at my workplace", i.e. some place directly related to work. If I were doing some remote work from home or café I would probably just say teen töitä (I'm working).
Olen työssä sounds like you are in the middle of some task, cf. olen työn touhussa. But as pointed out, it's not really that widely used and this comment has mostly curiosity value.