Osma took the reins of the point-less Manta from Juan Llop, but they remained as such through three games despite a brave showing against the unbeaten Mushuc Runa.
It took until the 63rd minute for the hosts to break through, with Juan Marcarie scoring the all-important goal at the Estadio Bellavista de Ambato.
Mushuc Runa moved clear with 10 points from four matches, although second-placed Emelec (seven points) have a game in hand.
Emelec powered to a 5-0 win at home to El Nacional, with Emanuel Herrera scoring a brace.
Herrera netted his first inside two minutes at the Estadio George Capwell, tucking a shot into the bottom left corner after the El Nacional goalkeeper had come off his line.
Miller Bolanos doubled Emelec's lead in the 10th minute, powering home a right-footed shot into the left corner.
In the 50th minute, Emelec made it three with some assistance from Dario Bone, whose attempt to clear a foray forward from the hosts ended in the back of the net.
Herrera and Fernando Gimenez added goals inside the final five minutes to wrap up the resounding victory.
Barcelona made it back-to-back wins with a 3-1 triumph at Deportivo Quito.
Ely Esterilla, Geovanny Nazareno and Ismael Blanco all netted for the visitors, before Quito's Jonathan Hansen put in a consolation goal.
LDU Loja salvaged a point from their hosing of Deportivo Cuenca, Danilo Carando's 88th-minute goal earning them a 1-1 draw.
Loja sit fourth on five points through four games.
Fifth-placed Independiente's hosting of LDU Quito was postponed.
