The 41-year-old leads by two shots over Frenchman Jean-Francois Remesy and by three over five others, including Darren Clarke, who posted 67s. His score of 64 was but two strokes off the course record of 62 set by Bernhard Langer in 1984.
'It was never going to count with preferred lies, but it still had to be done on a really tough course,' said Oldcorn.
'As soon as I didn't birdie the 15th I knew I couldn't match Bernhard's score, but I seem to do well on demanding courses.
Oldcorns birdie run commenced from holes No. 3 through No. 14, with his last five coming in a row. Should he hold on to the lead for three more days, he will collect his first win on the European Tour since the DHL Jersey Open six years ago.
'This course demands you play well off the tee, so that played right into my hands because it's my strong point.'
Perhaps the only other player in Thursdays opening round who had a more spectacular run of golf over El Saler was Clarke.
The burly Northern Irishman got off to a torrid start, making birdie on each of his first six holes, but then failed to do anything more but bogey his 16th of the day for 67.
Nevertheless, he is in contention for what would be his first win since last seasons Compass Group English Open.
This seasons Open de Espana has brought out plenty of Spains best as well, headlined on Thursday by Carlos Rodiles, who was among Clarkes group at 5-under for the day.
The trio of Sergio Garcia, Jose Maria Olazabal and Diego Borrego also put on solid opening rounds of 3-under 69. This week marks the first European Tour event in which Garcia and Olazabal have attended. They have been spending the majority of their time in the United States, and entering Friday find themselves just five strokes back.
Full-field scores from the Via Digital Open de Espana