Strange Problem

I have a 1996 Terrano/Pathfinder and I have a problem that seems to be giving some headache to solve. This is the situation.....The cylinders apparently are not giving the required compression....one set giving 150 approx and the other set 105 approx....the timing is correct...I recently changed a knock sensor or oxygen sensor......there is no sign of damage to the engine head (crack or blown gasket) .....what might be the problem and a possible solution
Grateful for any assistance

Thanks in advance
Tyrone