The U.S. requires money transfer services to be licensed under 31 USC 5330 in addition to any applicable laws at the state level. According to multiple sources online, including the thread referenced by MD-Tech's answer, OkPay either cannot or will not get a license, so they are out.
I dug on this a bit more because I thought it was interesting, and OkPay has other issues with U.S. and other regulators related to its interaction with Bitcoins, which themselves are a hot potato for regulation right now and may explain the licensing problem. It seems to also be facing regulatory pressure in other countries, by the way, so it's not strictly a problem they face in the U.S. Just for whatever reason, the problem is greater here. Some interesting summary points:
With mounting pressure on online money exchanges from US regulators, payments processor OKPay has announced that it is suspending processing for all Bitcoin exchanges, including industry leader Mt. Gox.
Earlier this month, the US Department of Homeland Security seized Mt. Gox's account with mobile payment processor Dwolla, on allegations that the account was in violation of US Code 18 USC § 1960 by operating an "unlicensed money transmitting business."
Just where the Bitcoin market falls under US law is unclear, because the legality of Bitcoin transactions has yet to be tried in court and law enforcement has refused to comment on ongoing investigations, such as the Dwolla case.
In March, the US Treasury said any firms dealing in the virtual currency would be considered "money services businesses" just like any other, which means they must hand over transaction information to the government and work to prevent money laundering.
In the UK, it apparently has also had trouble with banking partners (quoting a OkPay official regarding changing bank providers):
The UK bank that we used before did not make a final decision on whether to handle transactions in favour of crypto-currencies or not. Therefore the compliance department of the bank asked us to restrict such transfers.
This apparently allowed them to reverse a policy in the UK:
OKPAY's policy shift comes just months after it stipulated that GBP users check a box, verifying that their funds would not be spent on cryptocurrency, a feature that further incited users.
I hadn't heard of this company prior to your question, but having done some research, I tend to think that at least the part of this quote about language, attributed to a user, is true:
OKPAY are quite paranoid about AML and another problem is that their support people seem to be very bad at English, so their replies are often hard to understand. Their support are also slow [sic]. However in my experience they are an honest company.
I found at least one case where rumors that the entire company were going to shut down were traced back to a poorly translated message issued by the company.
Again, I know only what I read just now about this company, but it looked like there were a few red flags - the problems with the US probably not being the most important. This type of service is probably part of the future, but I'm not sure that I'd send money through it now in its current state or organization and regulation.