The pair were offered contracts along with seven other first-team players at a series of individual meetings at the Quarrier's Village training base on Monday afternoon.
Wideman O'Brien, 29, is now Ton's second-longest serving player behind Michael Tidser and has made 118 appearances and scored 22 goals since arriving at Cappielow in June 2010.
Goalkeeper Gaston, on the other hand, joined from Albion Rovers at the beginning of last season as back-up to veteran shot-stopper Alan Combe. Combe's subsequent departure to Hearts opened the door to rookie Gaston, who took full advantage and went on to make 42 starts - the most in the squad - as he cemented his place as No1.
Ton boss Allan Moore was delighted to tie the duo down for next term and particularly pleased with how enthusiastic they were about staying with the club.
He beamed: "David O'Brien and Derek Gaston have put pen to paper on new deals and we're delighted to get them tied up.
"I know what I'll get from wee Obri.
"He's very steady, never usually drops below a five out of 10, will get you goals and will always work hard. Good left siders are hard to come by.
"But I did have a wee chat with him and mentioned how he looked a bit jaded in the last four or five games - although, in fairness, he scored eight goals over the season and created quite a few.
"We think that might have been a wee bit of burnout, because there haven't been many games he's missed.
"That's something we might have to have a look at next season, having a bit of cover on the left-hand side so we can maybe rest him in some games.
"A couple of the boys are thinking about things, but Obri came in right away and said: 'I want to stay here, gaffer. I've worked with you long enough and I like the club.'
"That was a positive in itself."
Gaston, 26, was also eager to continue his fledgling career at Cappielow - and Moore believes he can take his game to another level this year after a positive first campaign as a full-time professional.
The Ton chief added: "Big Gats was happy to sign on the dotted line as well.
"This was his first season in training full-time and it's been a good one for him.
"He's got his faults that he'll have to work on, like his kicking, but his shot-stopping and overall play has been great for someone who has stepped up from part-time football."