how to open password protected 7zip archive on OSX

You can use the command line. One way would be to:

1. Install Homebrew
2. Install 7zip: brew install p7zip
3. Extract the archive in question: 7za x archive.7z

You will be asked to enter the password.

7-Zip (a) [64] 16.02 : Copyright (c) 1999-2016 Igor Pavlov : 2016-05-21
p7zip Version 16.02 (locale=utf8,Utf16=on,HugeFiles=on,64 bits,8 CPUs x64)

Scanning the drive for archives:
1 file, 520 bytes (1 KiB)

Extracting archive: archive.7z

Enter password (will not be echoed):
--
Path = archive.7z
Type = 7z
Physical Size = 520
Headers Size = 200
Method = LZMA2:12 7zAES
Solid = -
Blocks = 1

Everything is Ok

Size:       417
Compressed: 520