Some common expression for "oil and water don't mix"

[Silverobama]

What are some common ways to express "oil and water don't mix"?

A context: A guy called Alan reported to me (I'm the admin of some local English clubs) that a guy called Jack Wong was trying to take sexual advantages of young women and Alan thought that they were oil and water don't mix, so he reported what Jack was trying to do to me. How can I express this idea naturally or better?

How about "Alan and Jack are oil and water but don't mix"?

Please help me with a natural expression.

 

[Edward B]

"Oil and water don't mix" can be used when there are personality clashes, but not when a truthful report has been recieved.
It should not be used in the situation you described.

 

[emsr2d2]

What are some common ways to express "Oil and water don't mix"?

It's a set phrase so we don't have other ways of expressing it. If it's the right saying for a situation, we use it.

A Context: A guy called Alan reported to me (I'm the admin of some local English clubs) that a guy called Jack Wong was trying to take sexual advantages advantage (sexually) of some young women.

Note my corrections above. It's always "take advantage of", not "take advantages of". Note that I was going to remove the name of the person Alan is accusing but I have (for now) assumed you've had the sense to not use the real name of the accused. Since that person is presumed innocent until proven guilty, their real name should not be bandied around in public.

and Alan thought that they were oil and water don't mix, so he reported what Jack was trying to do to me. How can I express this idea naturally or better?

By using "oil and water don't mix" after "they were", you seem to be trying to use the whole phrase as an adjective. That doesn't work. You could say "Alan thought that they were like oil and water". People who know the full phrase will understand it. I don't really get what Alan was trying to say. Who are "they"? Is it Alan and Jack? Is it Jack and the women?

How about "Alan and Jack are oil and water but don't mix"?

No. See my notes after the previous quote box.

Please help me with a natural expression.

I can't. Edward B already explained that the phrase is used when people just don't get on (ie a personality clash). It doesn't fit this very serious context at all.

I hope you'll be limiting your use of the forum while you take on the almost full-time job of investigating this claim of sexual harassment and taking the appropriate action if it turns out to be true. You're going to be very busy with statements to the police etc.