The phrase "a combination of optimism" is correct and usable in written English.
You can use it when discussing the blending of different attitudes or feelings, particularly when optimism is one of those elements.
Example: "The team's success was attributed to a combination of optimism and hard work."
Alternatives: "a mix of positivity" or "a blend of hopefulness".
If Biden runs against Clinton, he will be motivated by a combination of optimism, habit, and fear.
With the players poised to strike, Showalter called his behavior at the time a combination of optimism, naïveté and denial.
Was there ever, except in the pages of Dickens, such a combination of optimism, collective industry, kindness, and pathos?
Leading shares hit their highest level for nearly seven months on a combination of optimism about Greece, rising oil prices and China acting to prevent a hard landing for its economy.
Along with those two, another necessary element is to have the right spirit: a combination of optimism, persistence and resourcefulness.
A combination of optimism about their futures and impatience to enjoy life now leads many to buy on credit.
Success, he said, requires a combination of discipline, optimism, humor, a willingness to share credit and good cigars and an ability to cut back-room deals.
Animated collectively by a combination of natural optimism and deep nationalism, Americans have made their president's campaign slogan, "Yes, we can", their own.
The turnaround is due, at least in part, to a combination of political optimism, support for infrastructure improvements from Britain and the European Union, and the proximity — in the boom years at least — of investors in the Irish Republic with credit to burn.
Mary Ann Tighe, vice-chairwoman of the real estate brokerage firm Insignia/ESG Inc. and a longtime friend of Mr. Silverstein's, described him as an "extraordinary combination of optimism and total shrewdness".
And yet, glib as I was, I still believe the American genius, for all its original sins (and slavery was a great sin), lies in a combination of an essential optimism and an essential pessimism about human nature so articulated by the nation's founders as to make self-correcting renewal the nation's core identity.
