The phrase "overly certain" is correct and usable in written English.
It can be used to describe someone who has excessive confidence in their beliefs or opinions.
Example: "While I appreciate your confidence, I think being overly certain about the outcome may lead to disappointment."
Alternatives: "too confident" or "excessively sure".
Osborne seemed overly certain that the "squeezed 10%" earning more than £40,000 a year would understand his argument about means-testing costing more than living with a few anomalies.
Gerry Eckber Bridgeport, Conn., Nov. 1, 2010 To the Editor: Re "Can the Dude Abide?," by Maureen Dowd (column, Oct. 31): Let me see if I understand this: Because President Obama appears overly certain of his convictions, he is branded arrogant.
An overconfident project manager is overly certain that his or her predictions are correct (Moore and Healy 2008) and thus will consider it unlikely that reality will deviate from her or his plans.
While perhaps overly certain of his own opinions on how to control the market forces of gentrification, Perks admits that this is an experiment.
The desire for definitive information on pressing issues of the day can foster a culture of short, overly certain messages that seem to change over time.
Inability to recognize when one is making an error or a tendency to be overly certain about one's interpretation of events might play a role in the emergence of psychosis.
Though Jonah (or, in the words of Mr. Klain, "being an obnoxious, pompous jerk") may be the prototype that has most pervaded the Washington lexicon, almost every character on the show can serve as a convenient nickname for a certain overly eager, overly machinating and overly self-promoting Washington staff member.
Gibbs acknowledged that there were times in the campaign when he became overly aggressive with certain reporters, which he regrets.
"One of the dangers for our political opponents is not to seem overly exultant when certain reports are not as strong as predicted," Mr. Plouffe said.
But in translating it to the stage he has become overly obsessed with certain logistics of plot that require lengthy, ultimately tedious explanations by the two leading characters.
Potential buyers should at least check a fund's portfolio to make sure it isn't committed to more low-quality risk than they want and isn't overly concentrated in certain industries.
