It depends, dominator it's way better in olympiad then MM or SS. Every character it's good in olympiad if you know how to use it properly. So, if i were you for olympiad if i wanna kill a lot i will go for dominator.
A support is always welcome by everyone , especially when we talk about domi but as for the oly part ( btw both are awesome for oly and pvp ) things are not so easy.
If i was now to explain the reasons that these two chars are hard in oly , it would start another flame thread and im kinda tired of drama(rich)queens.
This period ( and probably for long time ) sps and domi are captured.
I'm not saying that the current player[STRIKE="(s)"][/STRIKE] that have these heroes are hard to be beaten , but behind them there are two former legendary players that it just happens now to leave oly for some reason , so if they turn back , anyone who have sps and domi will lose it next month.
Maybe focus on something else , dagger is a good class too or maybe soulhound.