If I send messages using OTR & GPG, will my ISP or some surveillance
agencies know that I'm using them? "Look, he's sending an encrypted
message. He must have some important secret."
Yes. The NSA and their friends are reported to look for and archive encrypted messages wherever they find them, presumably on the basis that if it is not encrypted, they can check if it is suspicious right away, but if it is encrypted they had better keep it in case it is important later. Before Snowden, we kind-of knew that they could do this, and presumably did do this whenever they had a good reason. But now we know they do it all the time and against everyone, without any reason at all.
So yes, if you talk about suspicious stuff you will come under (mild) suspicion. If you use encryption to hide what you are talking about you will also come under (mild) suspicion.
You won't be rubber-hosed without a particular reason, because the rubber-hose guys need wages, whereas just storing all your email and browsing history forever is very cheap. If you become a problem, they have got your complete web browsing history though (so they can blackmail you by exposing your porn/gambling/drug/other habit), as well as the capability to plant incriminating files on your computer pretty much at will.
But don't worry! They won't use any of that against you.
You have nothing to hide.
Because you are a good citizen. You don't commit Hate Crime. Or Thought Crime. Or Pre-Crime. You are not a subversive, are you? You don't have opinions they don't like - or if you do you keep them to yourself! You don't cause social unrest do you? You don't talk to people and make them dissatisfied with the current situation. You don't get involved in political campaigns to change government policy.
So you have nothing to fear.
Big brother is watching you. But Big Brother loves you.
And you love Big Brother, don't you?