Mostly all players of DotA2 knows this item.
Sometimes it said as “yellow” ward.... (easier while chat), you can get this item
by cost 150 gold - it's equal 3 / 4 / 5 creeps killing.
Even It's cheap, but it limited by the time (stock cool down) if the max stock 2 had bought.

Detail profile of this item and other rules apply to this item could be read on Observerward[dota2.gamepedia.com]

Because limitation stock of Observer ward, this item so important to support our team
while attack and defend. Mostly this job should take by support role hero for buying
and put it on there. I don't know if one of our team were kindly help do that.... :) - lol

How much observer ward we need while attack and attacked near the barrack?
Three (3), its enough and make sure put it on the right place.
So you can see the enemy easily. #2 of them put on top of high rock (temple – radiant area),
and the other one put on middle lane.

While defend, observe the trend of attack coming from.
If it come from bottom, so you need put on bottom rock and middle lane. If they come from top, put the observer ward on top rock and middle lane. If you have more stock, put the observer ward on that 3 location because the attacker usually change over from middle, then top, continue to bottom and vice versa.

What about if your team are pushing / attack?
Ask your team focus of which tower to tumble down. If they say top, put on top rock (temple) and middle lane. If they say bottom, put on top of bottom rock and middle lane. But my mind thinking if you attacking to enemy barrack, it will harder put 3 observer ward. It will better support your team for rush and war.

Remember, while attack put the observer ward is out range of enemy tower or your ward will be useless.


Be happy as support and warding....:)

Once again. This is my perspective. Sorry if you be disagree. I am the newbie who describe my mind about observer ward. Dont forget put sentry on stone, cause your enemy somotimes put obs wad also on there.
This article is appear based on what I have done as full support as shown on my DotA2 profile.
I don't know if it come similar with others.

My english not truely as good as best writer. I hope you understand what i mean on this article.
Any opinion and constructive comment for better is advise. Thank you for your time to read it.
Ransom - Satisfaction
DhiaNet
Jambi - Indonesia

Also thanks to these sources.
Gamepedia[dota2.gamepedia.com]
Dota2 Map - Interactive[devilesk.com]