Only if

Only if is used expresses the meaning of "in case" or "only in the case". You need to differentiate between "only if" and "if only".

For examples,
We will cancel the picnic only if it rains. [if there is storm, sunny, windy…., we will have a picnic. So, the cause that can make us to cancel the plan is only rain.]
Call me only if your cold gets worse. [Only when your cold get worse, you can call me. ]
Only if everybody agreed would I accept this position. [I would accept this position only if everybody agreed.]