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.]