Macbook Pro 2014 and "Replace Battery Soon" issue.

You can try running the Diagnostic/AHT http://support.apple.com/kb/HT1509 It may or may not kick out an error code.

hold the option D key on reboot. Have your wifi psswd handy at that time.

Even a 2014 and no warranty you can still get a free over the counter 'Apple Service Diagnostics' test to assess your hardware: magsafe connector, DC-in board and on the logic board the Controls T1 and T2 and inline f1 inline circuits.

Make an appointment—

https://www.apple.com/retail/geniusbar/

From iOS online Genius Bar appointment:

https://itunes.apple.com/us/app/apple-support/id1130498044?mt=8

Yes, but you're not going to like it. If your Mac is supported (and it is) you should only have Apple replace the battery. Having performed that operation yourself, an Apple Store will henceforth refuse to service it under any circumstances, or any reason.

As you suspect the problem may be external to the battery. Go ahead and ask, but I think the best you can hope for is for the Genius Bar to refer you to an Apple Authorized Service Provider. Depending on the particular facility, they may have more discretion.

Otherwise contact OWC again and repeat the exercise for as long as they are willing. OWC is a reputable vendor, but unlike Apple their liability is limited to the parts they sell.

You say it is a MBPro 2014, but do you really mean it or is it a Mid-2012 model which is the last one with a user replaceable battery? Click on the Apple in the upper left side of the menubar and select "About this Mac".

If there happens to be a manufacturing defect, it can affect a whole shipment that OWC might have. It is also possible that these third party batteries may not be 100% compatible with how Apple determines battery health. (I know OWC usually pretty good about making sure their products are compatible, but maybe Apple changed something in the OS.) If the battery is not at fault, then it will most likely be a Logic Board issue.

What does the Apple Diagnostic/AHD report as leroydouglas suggested?

I'm having a very similar problem with an OWC battery. I'm on my second and the third is on its way. My Macbook Pro is a mid-2012 Retina. This model has a glued-in battery and required almost complete disassembly to do the battery change. That process took me about 3 hours and I was not too far into it before I regretted not taking it to Apple and paying whatever they wanted. The second time was a little easier. After the second one failed I called Apple and, long story short, they don't make these batteries anymore and referred me to an authorized service center. I called them and they use these same OWC batteries. The service center told me they do have occasional problems with them but they are rare. So OWC may be the best option -- I'm going for three and hoping for the best.

In each case after a couple months the battery capacity started decreasing and within a few months I had the "Replace Battery Now" warning. I monitored the second one with coconutBattery -- after 30 cycles the battery capacity started decreasing at 6% (almost exactly) with each charge cycle. When the capacity reached zero it rolled around to a large number suspiciously close to 65535, so it appears the defect may be more with the battery's firmware rather than the battery itself. The battery isn't the greatest but it is far from dead. The idea that OWC has a bad batch of batteries seems plausible and maybe a firmware update will fix the problem.