Oxidation

Still new to this and made a fatal error when brewing my Octoberfest. When transfering from the primary into a secondary I realised I was short about a 1/2 gallon of liquid. Not thinking, I added spring water into the beer, and only then thought to myself that I may have introduced oxygen into the beer. How worried should I be? Any advice?

Not too worried.

Stop worrying about volume.

Stop adding liquid to your fermenters during fermentation.

Chances are you didn't oxidize your beer but you definitely diluted it.

Guess it depends on how careful you were pouring in the water. Besides oxidation, you really watered down your brew. Nothing you can do now but wait and see.....

Only time will tell.

An Oktoberfest should be fine. There are compounds associated with darker grains that help inhibit oxidation.

Just don't do it again...

Thanks for the replies. The only reason I added the water was the recipe I formulated was for a 5 gallon batch. I had meant to top off the primary at 5 gallons and simply forgot. I was very careful about adding the water, using a syphon, so as to create as little splashing as possible. I'm just nervous because I really don't want to put a huge amount of time into a beer that will build lots of anticipation by October only yo find it's not really very good. But those are the chanes you take I guess.

Even if the recipe was formulated for a 5 gallon batch, don't feel the need to automatically top it off. Get into the habit of checking the gravity with a hydrometer, and then add water or malt extract if needed. I know this was kind of an after the fact kind of addition, but even when I go back to the occasional extract brew I still check the gravity just for good measure. I know plenty of people who never check gravities and make great beer, so take my advice with a grain of salt. I personally just like controlling as many aspects of a brew day as I can, so that my brews are consistent. Oh, and addressing your first question, you should be fine unless you sprayed it in with a pressure washer.

Here's a great calculator for adjusting gravity if you don't already have a brew app.
http://ahomebrewlog.com/gravity_adjustment/

next time make more beer to compensate for the loss due to racking.

+1 If you want 5 gal going in the bottles then shoot for 6 gal at the end of the boil if you are doing full volume boils. That way you assume 0.5 gal lost between kettle and primary and another 0.5 gal lost between primary and bottling bucket. Personally, I always end up with 4-4.5 gal going into the bottling bucket due to lower boil volumes.