Since the reply to my query was so overwhelming, [cough, cough], I feel my patience was tested to the limits at times with using MikroTik. But I love its value and full of features. I also wish some gentle push with appropriate efforts by MikroTik to make its products even more valuable in terms of finding out its features in a more systematic way.
I know my MCNA qualification does not mean I know all about MikroTik products. But I do own a copy of excellent 'RouterOS by Example' by Stephen R.W. Disher, as well as first edition of 'Learn RouterOS' by Dennis Burgess. But still found myself many times stumbling in the dark (so to speak) by lack of documented features by examples. Please do not see this as a complaint.
(I know of a certain IT company which asks chosen beta testers to help develop its product such as debugging and documentation before the product is released. The price to the company is relatively cheap: Beta testers got to keep the products they test. That's simple. I am one of their beta testers so I know but I can't name name.) [End of unsolicited advice]
Back to the issue at hand, I finally resolved the issue after a lot of perseverance and time. I am glad to say that I can now connect to MikroTik (RB493G) via OpenVPN from my Android devices - one is my HTC Desire (h/ware is a bit old) running modified Cyanogen ROM (4.2.2); the other is my Acer A500 (also a few years old) tablet running modified Omni version 4.4.4. I used to connect via PPTP but it can be easily cracked (source Edward Snowden).
For those who are interested, I have to set OpenVPN server to IP mode for Android devices to connect. Whereas Windows can only connect if I set it to ethernet mode. In short, I can only connect one type of devices at a time but not Windows and Android at the same time. In addition the following script to make it to work (Android only) along these lines:
remote pubic-ip-adress-of-Mikrotik-router (or your DNS name)
#comp-lzo # Do not use compression.
route local-network-ip-adress netmask ip-adress-of-ovpnserver
# additional subnets on your local network s/be added here in the same format if you want access when connected.
P.S. Now the question 'How to have Windows and Android connecting Mikrotik openVPN server at the same time? Or is it possible?
Over to expert. Anyone at all.