Oasis Outsourcing Holdings Inc. agreed to pay $5.99 million to settle claims that it allowed excessive fees in a multiple-employer 401(k) plan it administered for its clients and their employees that, at one point, had up to $1.2 billion in assets.

The multimillion-dollar agreement comes two years after the workers brought an internal claim against the plan over alleged imprudent investment options and excessive fees paid to Lincoln Financial Group, the plan’s former administrator. The workers Aug. 17 filed a lawsuit along with settlement papers in the U.S. District Court for the Southern District of Florida.

This is the second...