The expiration of the Hart-Scott-Rodino waiting period is a significant regulatory milestone, indicating progress in the acquisition process. This development is likely to be viewed positively by investors as it reduces regulatory uncertainty surrounding the deal. However, the transaction's completion is still subject to other regulatory approvals and customary closing conditions, which introduces some level of uncertainty. Overall, this news is likely to have a positive short-term impact on Aon's stock price, reflecting investor optimism about the strategic benefits of the acquisition.