MacDonald cancelled out Gozie Ugwu's early strike with a cool finish on the stroke of half-time. He then set up strike partner Danny Philliskirk for Oldham's winner 15 minutes from time.
"Charlie will always cause a problem and actually his movement's fantastic," said Johnson, whose side have now won four games in a row in all competitions.
"It was a really good finish and we knew that was going to happen and we knew we would get at least one chance for him.
"(In training) we did a bit of work, one v one with the keepers, and Charlie MacDonald scored eight out of 10. He's just ruthless, clinical in that area. I was delighted for him to put that in."
Johnson added: "I am pleased because we've won ugly and that is an art as well. We've had a really good run lately."
- Sports & Recreation
- Danny Philliskirk
- Charlie MacDonald
- Gozie Ugwu