Vassell joined the Latics from Stoke last season and worked with Saddlers coach Dean Holden at Boundary Park.
"I think this will be a really good opportunity and hopefully I can get some game time," the 19-year-old said.
"We've got a lot of young players and a few who are more experienced. I feel I've settled in well."
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Walsall have signed centre-back Theo Vassell from fellow League One side Oldham Athletic on a one-year deal with the option of a further year.