We achieved organic revenue growth of 10% in the fourth quarter, with double-digit growth in commercial risk and reinsurance, driven by net new business generation and client retention.
Our full year organic revenue growth of 9% reflects the strength and momentum of our Aon United strategy, which is designed to drive top and bottom-line results.
To that point, operating income increased 17% year over year.
Full year operating margins expanded 160 basis points to 30.1%, with margins of 32.8% in the fourth quarter, reflecting ongoing efficiency improvements, net of investment and long-term growth.
Earnings per share increased 22% for the full year, free cash flow exceeded $2 billion and we completed $3.5 billion of share buyback in 2021, a strong indication of our confidence in the long-term value of the firm.
This work has been formed by 20 years of enrollment data from over 4 million participants, which enables us to rapidly develop bespoke solutions for our clients that strengthen their total rewards offering and reinforce their human capital strategy at a time when this has never been more essential.
In the quarter, we delivered 10% organic revenue growth, the third consecutive quarter of double-digit organic growth, which translated into double-digit adjusted operating income and adjusted earnings-per-share growth, continuing our momentum as we head into 2022.
As I reflect on full year results, first, organic revenue growth was 9%, including double-digit growth in commercial risk solutions and health solutions.
I would note that total revenue growth of 10% includes a modest favorable impact from change in FX, partially offset by the impact of certain divestitures completed within the year.
We delivered substantial operational improvement, with adjusted operating income growth of 17% and adjusted operating margin expansion of 160 basis points to a record 30.1% margin.
The investments we have made in Aon Business Services give us further confidence in our ability to expand margins, building on our track record of approximately 100 basis points average annual margin expansion over the last decade.
We translated strong adjusted operating income growth into double-digit adjusted earnings per share growth of 22% for the full year, building on our track record of double-digit adjusted earnings per share growth over the last decade.
As noted in our earnings materials, FX translation was an unfavorable impact of approximately $0.03 in the fourth quarter and was a favorable impact of roughly $0.23 per share for the full year.
If currency will remain stable at today's rates, we would expect an unfavorable impact of approximately $0.16 per share or approximately $48 million decrease in operating income in the first quarter of 2022.
In addition, we expect noncash pension expense of approximately $11 million for full year 2022 based on current assumptions.
This compares to the $21 million of noncash pension income recognized in 2021.
In 2021, free cash flow decreased 23% to $2 billion reflecting strong revenue growth, margin expansion and improvements in working capital, which were offset by $1 billion termination fee payment and other related costs.
I'd observe that excluding the $1 billion termination fee payment, free cash flow grew $400 million or approximately 15% from $2.6 billion in 2020.
Given this outlook, we expect share repurchase to continue to remain our highest return on capital opportunity for capital allocation as we believe we are significantly undervalued in the market today, highlighted by the approximately $2 billion of share repurchase in the quarter and $3.5 billion of share repurchase in 2021.
We expect an investment of $180 million to $200 million.
We ended 2021 with a return on capital of 27.4%, an increase of more than 1,500 basis points over the last decade.
In addition, we issued $500 million of senior notes in Q4.
Our net unfunded -- funded pension balance improved by nearly $500 million in 2021, reflecting continued progress and a result of the steps we've taken over the last decade to derisk this liability and reduce volatility.
We returned nearly $4 billion to shareholders through share repurchase and dividends in 2021.

Summary:
We returned nearly $4 billion to shareholders through share repurchase and dividends in 2021.