How to use the slang phrase, โอปะเตง
Two of my Thai friends told me about this phrase, but they each said that it means something different. Neither one of my friends is a super advanced English speaker, so maybe one of them lost the meaning in translation.

Friend 1 said that it means "Are you ok?"

Friend 2 said that it means "Is that ok?" or "Is that ok with you?"

These phrases sound similar, but mean something quite different. So, who is right? Thanks in advance!

Question

How to use the slang phrase, โอปะเตง
Two of my Thai friends told me about this phrase, but they each said that it means something different. Neither one of my friends is a super advanced English speaker, so maybe one of them lost the meaning in translation.

Friend 1 said that it means "Are you ok?"

Friend 2 said that it means "Is that ok?" or "Is that ok with you?"

These phrases sound similar, but mean something quite different. So, who is right? Thanks in advance!

Answer

My English is not in advanced level either but I will try my best. 
If I'm not wrong "โอปะเตง" or "โอป่ะเตง" should be a slang/omitted for "โอเคไหมตัวเอง" that has the same meaning as "ตกลงไหม?" which it simply means "OK?" in English.

literally meaning  => โอ(โอเค/ตกลง) OK + ปะ(ไหม) is it? + เตง(ตัวเอง) you

e.g. วันนี้(เรา)ไปดูหนังกัน โอปะเตง = Today, let's go to the movies (together), OK?

Answer

For me I guess both of them are right.

"โอปะเตง"  is slang as pervious answer told you.

The sentense "โอเคไหมตัวเอง" can be divided into 2 parts. "โอเคไหม" and "ตัวเอง" and they are simply translated to English  as "Ok?" and "YOU".

for "Ok?" did not mention that WHAT is OK? so it can refer to "YOU" or something else.
            if "ok?" refer to "YOU" the meaning should be "Are you ok?"
            if "ok?" refer to something else  the meaning should be "Is that ok?"

for "YOU" in this sentense is used to mention that I'm talking to "YOU".

Answer

โอป่ะเตง = โอเคหรือป่าวตัวเอง 
โอป่ะเตง is slang

โอ = โอเค(OK) 
ป่ะ = หรือป่าว?(rue-páw?) 
เตง = ตัวเอง(tua-ang)

โอป่ะเตง(Slang Thai language )= โอเคหรือป่าวตัวเอง (Thai language )= Are you OK? (English language )

Answer

I never heard my friends say "โอป่ะเตง" to each other narmally they use this phrase to only boyfriend or girlfriend  "โอ" is from Okay in english / "ป่ะ" is use for the question / "เตง" is from ตะเอง or ตัวเอง it's mean babe, darling or honey, so we ues "โอป่ะเตง" for only boyfriend or girlfriend  and it is mean Is that OK, honey?

If you want to say "Are you OK?" You should say โอป่ะ?

Remember ตะเอง, เตง or  ตัวเอง are mean honey, babe or darling 
Your second friend is closely right^^

(My english is not well but I try to explain my best)

Answer

It's informal phrase which you may use to your closed friend. 
For example, when you want to ask your closed friend to the zoo and you already planned for every things and explain all detail to your friend then you ask your friend that โอปะเตง to ask for his or her opinion that if he or she ok with your plan.

Or when you try your dress on and wanna ask your boyfriend or other closed friends whether that dress fit to you or not.

On the other hand, this phrase is also be used when your closed friends or your boyfriend get in trouble or not feel good in order to ask them if they are ok or alright.

However, you have to switch your feeling sound when you say this phrase which one is happy sound and second is sympathetic sound.