Cannot be exercised prior to expiration. Understand also that OTC dealing is a dealer's mkt, and will lack transparency. Nadex is the only outlet for OTC FX in the US.
CME FX Options; both american and european style. Not sure where everyone is getting OTC from.. OP never mentioned OTC.
Well he stated broker, not exchange. I assumed he wanted options on spot. In any event he can trade CME or the ETFs.
He said forex. To me, "forex" means OTC forex, as opposed to exchange-traded currency futures and options on them. I do see that the exchanges are starting to use the term "forex" in marketing their currency futures and options as well. I think they are just trying to take business from the (much larger and more liquid) traditional forex market by mixing up the terminology. Their brokers are certainly not forex dealers as the term is used by many, including the CFTC. But yes, you can also trade options on the (fixed size and expiration) currency futures at CME, and other currency options on a couple of stock exchanges.
For FX options. on an exchange In smaller sizes (ie mini lots up to 200 at a time etc..) i like the Nadex spreads. But these are for short term intraday to 1 day trades only or for hedging like buy eur/usd sell a spread or sell eur/usd and buy a spread. So this may or may not suit your purposes.