Oanda spread sucks after Wall Street Close. Shown here Current OANDA Spreads and Pairs - OANDA FXTrade
it’s 2 pips as I write.
Whereas IamFX is 1 pip. Any other brokers offering good spread after market close ?
Actually, Oanda’s spreads are largely typical. I have a bunch of different versions of MT4 and all of them have spreads varying from 1.5 - 2.2 pips right now.
Maybe check what they are during London open.
Why do they have such large spread whereas others don’t ? Is it to wipe out all daytraders at the end of a day 
It fluctuates for changes in volume, news releases, etc. I trade on large timeframes so a difference in a few pips means nothing for me overall. However, if you’re looking to trade Oanda when the spread is narrow, you’ll need to look during a time when activity is the highest. I suspect that would be during New York, London opens, as well as when London and New York trading overlap.