Otto finished alongside Martin Erlandsson, Julien Clement, Raphael Eyraud, Neil Cheetham, Per Nyman and Birgir Hafthorsson atop the leaderboard at Aa Saint Omer Golf Club.
Otto, who finished in the top 10 in the 2003 British Open, ran off back-to-back birdies starting at the par-4 third and jumped to 4 under with an eagle at the par-5 seventh.
The South African kept on rolling with a birdie at the par-5 ninth but he found trouble with a double bogey at the par-4 10th. Otto recovered with a birdie at the par-3 11th and moved to minus-5 with a birdie at the par-4 15th.
Otto parred the 16th but suffered the first of back-to-back bogeys at the par-3 17th to close out his round.
'I hit myself in the wrong places twice on the 17th and 18th and had putts up the slopes and down the slopes so if you hit it in the wrong place then you have to fight for it here,' said Otto. 'But I would have taken a 68 this morning before going out there but it's four rounds and tomorrow is another day.'
Clement dropped a shot at the par-5 14th to fall back to 1 under but he countered with birdies at the 16th and 17th holes for his share of first.
Nyman had two bogeys and a birdie over his first four holes but he added back-to-back birdies from the seventh to get into red figures. Nyman tallied a pair of birdies on the inward half to finish at 3 under par.
Erlandsson started on the back side and picked up a lone birdie at the par-5 14th to make the turn at 1 under. He added a birdie at the first and reached 3 under with a birdie at the fifth. Erlandsson stumbled to a bogey at the following hole, but came back with a birdie at the ninth for a 68 of his own.
Hafthorsson birdied three of his last five holes for his share of first while Eyraud struggled to a double bogey at the par-4 eighth that dropped him back to 2 under. Eyraud responded with a birdie at the ninth, his last, to join the logjam in first place.
Jamie Little took the lead with a birdie at the par-4 15th but he bogeyed each of his last two holes for a round of 69. Little was joined by Ivo Giner, Diego Borrego, Jamie Elson, Phillip Archer, Craig Williams, Michael Jonzon and James Hepworth at 2 under par.