It's already known that Obama tapped the phones of a Fox News White House reporter and German Chancellor Angela Merkel. Considering Democrats made every single attempt to defeat Trump and failed, Obama illegally tapping Trump Tower doesn't surprise me in the least bit.
To summarize, tapping the phone of a foreign leader is an utterly different act legally speaking than doing so for a US citizen who was the Presidential nominee for a major party.
Whatever else you are going to says about the Fox News reporter situation, it was at least done properly in that a court warrant was obtained.
A basic question in this case is that Trump would have immediate access as President for absolutely any court warrant of this type so where is it? (And significantly a court would be highly unlikely to approve a warrant in this specific situation given the potential political complications barring very strong evidence of criminal behavior being presented first.)
Incidentally, in a hypothetical world where somehow Obama used private citizens or acted completely outside of the law or the regular chain of command, Trump certainly still should not be tweeting about it and should have the FBI aggressively investigate the case and say nothing until court criminal indictments are issued and those involved are being arrested.