A succession of knee injuries has meant full-back O'Brien's last senior appearance for Bolton was in October 2008, although he did figure in pre-season last summer.
Midfielder Gardner has also been plagued by a knee injury this season, restricting him to just one appearance, in a Carling Cup win at Southampton towards the end of August.
"Joey and Ricardo played an hour of the game which is great because when you've been out for a period of time, as both lads have, there is a feeling of frustration," said Coyle.
"But when you're back on the pitch running around again with your colleagues, then it's terrific because they know they're on their way back to recovery. They both did very well and are progressing nicely, which augurs well.
"But they will know they are a number of weeks away from full fitness and we will give them all the help we can to get back. I would hope they will both be back before the end of the season, although they have to make sure they are 100% fit before they can show everybody what they can do."
Source: PA
Source: PA