Established: 2001 (as a Forex broker)
Country of Origin: Canada (with branches in the USA, Japan, UK, and elsewhere)
Regulated: Yes (NFA and CFTC in the US, IIROC in Canada, FCA in UK, JFSA in Japan)
Accepts US Traders: Yes
Business Model: STP with internal netting of smaller trades
Minimum Trade Size: 1 unit (single unit of base currency)
Trading Software Platforms: FX Trade (OS-independent Java software), MetaTrader 4 (MT4)
Commissions/Spread Cost: Competitive STP-style widened spreads
Best Suited For: Beginners, long-term less frequent traders, Americans and residents of most developed countries
Official website: fxtrade.oanda.com
OANDA began in the pioneering era of retail Forex. That is, they were basically structured as a bucket shop initially. As of recent years, they claim to net smaller trades together and pass them on to their liquidity providers to avoid the traditional conflict of interest issues. Either way, whether they take a little profit off the table on occasional (from losers who will lose either way, in most cases) they do seem to have maintained a very respectable image by not unfairly manipulating customers to lose.
Unlike some of their peers from the first wave of online Forex offerings, OANDA's business model revolves around attracting high volumes through relatively low spreads (compared to the group they actively compete with, at least) and have grown enough capital to become a fully regulated broker in at least the US, Canada, and UK. (The situation in Canada is a little complicated now though, since the genius politicians in each province can't agree on what's up and what's down when it comes to "protecting" their voters from free will choices.)
OANDA's greatest claim to fame was to become the first retail broker to offer single unit trade sizes. For professionals, this is pretty much a non-issue since even a percentage-based money management strategy can be done with larger lot sizes, but it allows under-capitalized beginners to run the same kind of money management in terms of percentage.
Unfortunately, if you're a frequent intraday trader (especially if you're a scalper), OANDA is notorious for artificially widening spreads to the tune of 20 pips or more during volatility spikes. This pretty much disqualifies them as a broker for news traders and anyone who holds tight stops near major economic news announcements.
Frankly, you can't blame them. They take on a lot of risk to offer the kinds of average spreads they do (which, of course, works out well for them most of the time) but their business model can't afford to risk offering the same standards of pricing around extreme volatility conditions.
They also used to be the only broker in the world to offer weekend trading... which, admittedly, wasn't all that practical unless you were an intraday trader who fell asleep on Friday afternoon holding a sizeable position and desperately wanted to flatten any exposure to Sunday's open (and, for some reason, didn't mind paying the enormous weekend spreads they offered.) We can't fault them for getting rid of it.
They've always offered a Java-based platform that runs equally well on Mac OS and Linux as it does on Windows, but they finally gave in and began offering MetaTrader 4 alongside it a few years back. So far, their MT4 offering is one of the better implementations of the platform with no obvious signs of manipulation from dealer plug-ins or similar systems.
Deposits and withdrawals are generally processed promptly. They also happen to be one of the few brokers in the world that still offer PayPal so that could be a plus for you if you don't mind the usual PayPal fees. Unfortunately, they had to pull the plug on PayPal funding for American customers in 2015 as part of an oddly arbitrary and illogical regulatory change. (Non-Americans can continue using it for deposits and withdrawals as of this writing.)
Overall, if you're trading a relatively small account (especially below $500 in risk capital) and your style is medium to longer term swing trading (and you don't typically focus on intraday positions with tight stops around big economic news announcements) then OANDA is one of your best choices. They're a dream for stop-based money management down to the dollar on smaller accounts for beginner and intermediate swing traders.