Joe,
I suggest a direct and non-paranoid conversation with someone in HR at your company. As you probably know, the commissions on A shares vary with the size of the purchase. They generally start around 5.5 percent for the minimum purchase but decline as the size of the commitment increases. Many funds have ZERO commission on purchases of $1 million or more. So you really have to ask your HR person what the company negotiated with Oppenheimer.
Small plans generally have higher expenses than large plans and some companies get record keeping, etc. bundled into the plan by allowing such higher expenses, including commissions.
As a general rule, I've always found HR people to be sincerely interested in getting the best deal for employees. Sadly, many are not very sophisticated so they can be sold a bill of goods by the people who sell plans. Worse, HR people are generally over-ruled by higher ups who may be calculating the impact of higher company expenses on their stock options. In other words, it's better to be world-wise than suspicious.
Scott