The 6ft 2in centre forward has signed a two-year deal and becomes Phil Brown's seventh addition this summer.
Dalby made 18 appearances and scored one goal for the O's, which came on his full debut in a League Two fixture against Crawley Town.
In January 2018 Leeds United completed the signing of the youngster for an undisclosed fee and, after initially impressing in the Whites' U23s, Dalby featured for Marcelo Bielsa's first team in pre-season and was on the scoresheet for the first time against York.
He joined Morecambe on loan in January 2019 before Watford snapped up the forward later that year for another undisclosed fee.
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