The one-way ticket will arouse suspicions not of "visa overstay" but of "what are you planning to do in Spain?".
Part of the profiling process involves when you bought your plane ticket, whether it's open-ended, one-way, etc. Government agencies don't just look at country of origin and skin color, and never have. I work with them, I know...
If your intention is to not arouse suspicion (one who is traveling to another nation with the expressed purpose of violating visa regulations would probably choose to stay as low-key as possible), I suggest you purchase a RT ticket well in advance. It will most likely spare you undue headaches further down the line, in say, six months
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Contact the airlines, as unused portions of RT tickets are usually redeemable for standby flying, although the charges for standby on US carriers have recently been increased (I believe they're at about $100).