In May 2014 the Angels players started forming their hands into an “O” shape whenever a player made a great hit. They also started using the hashtag on twitter #opentheroof. You can see it in this tweet from Tyler Skaggs. It seems to be a thing between CJ Wilson and Tyler Skaggs.
— Tyler Skaggs (@TylerSkaggs37) May 25, 2014
The oldest reference seems to be this one from CJ Wilson on April 5 following a closed roof game in Houston.
— C.J. Wilson (@str8edgeracer) April 6, 2014
It’s most probably a call for open air stadiums.
As for the “O” hand gesture it appears to be a way of saying “We Are Together”. Mark Gubicza confirmed during an Angels broadcast on June 19th that the team is declaring their unity by bringing their hands together as a whole.