Pia Christmas-Moeller, the special co-ordinator of the observer mission, said: "I'm happy that election day took place in a generally good atmosphere, but I'm even more disappointed that these elections failed on many basic democratic standards."
The OSCE report Monday also said state television news coverage of the campaign was minimal, limiting voters' ability to make an informed choice.
Tajikistan, Central Asia's poorest country, voted on Sunday.
Official results have yet to be announced but the vote is widely expected to cement the grip on power of Emomali Rakhmon, the incumbent president, who has ruled for almost two decades.
The main opposition Islamic Revival Party says it has evidence of fraud.
"The past elections in Tajikistan had nothing to do with democracy," Khikmatullo Saifullozoda, one of the party's leaders, said.
Commenting on irregularities, the central election commission said on voting day that violations were not being reported on a large scale and would be addressed.
Preliminary election results are expected later on Monday.