The pair join Blackburn forward Jordan Slew, who signed on loan from Blackburn in time to make his debut in the New Year derby win over Inverness.
Former Celtic youth-team player Tidser has joined on loan until the end of the season. The 23-year-old has made 16 appearances for Sky Bet League One side Rotherham since joining from Morton in the summer, but most have been off the bench.
Left-back Oikonomou joined County last January and played 18 games, losing just four times, before returning to his homeland in the summer to join Super League side Veria.
The new arrivals, who could feature against St Johnstone in the Scottish Premiership on Saturday, coincide with the departure of striker Orhan Mustafi, who made a minor impact during his loan spell from Grasshoppers Zurich.
Manager Derek Adams saw his squad further bolstered over the festive period by the return of Alex Cooper, who netted his first goal for the club during the 2-1 win at Inverness, which ended a 10-game run without a victory.
The 22-year-old midfielder was making his 30th appearance for the club but the former Liverpool trainee had only started one game previously this season.
"I was disappointed to miss the first half of the season through injury but I feel I have got myself back in very good condition now," Cooper said.
"I got an injury in the first week of the season to my foot, which escalated and dragged on longer than maybe it should. It turned out to be swelling between the bones.
"The physio said he had never seen anything like it before, but I saw a specialist and he sorted it out quite quickly.
"I just want to do as well as I can now and kick on. It's in my hands, hopefully I can do well enough to stay in the team.
"I was patient and kept training away and got myself as fit as I could to take my chance, and I just want to take my chance whenever I get it."
Adams was delighted with Cooper's contribution going forward and tracking back with Caley Thistle full-back Graeme Shinnie.
Adams said: "He's a boy who works really hard on his game. He is always doing extra, he has a great work-ethic."
Meanwhile, Adams has earmarked St Johnstone central defenders Steven Anderson and Frazer Wright as the main obstacles when the Perth side visit Dingwall.
"They have got Anderson and Wright who are not the prettiest of centre-halves. They are ugly, they are warriors. They like the physical side of games, so we need to deal well with these two."
County have doubts over defender Steven Saunders (hamstring) and midfielder Stuart Kettlewell (Achilles) ahead of the fixture against St Johnstone.
Saints have signed former Scotland Under-21 striker Michael O'Halloran on an 18-month contract.
The 22-year-old has secured his early release from Bolton after failing to make a first-team appearance for Wanderers.
St Johnstone, who are without Patrick Cregg (suspension), Brian Easton (hamstring) and Steven MacLean (knee), say O'Halloran is likely to feature in their squad to face Ross County.
The signing allowed manager Tommy Wright to loan prolific under-20s striker Chris Kane to Dumbarton for one month.
Meanwhile, another young Scottish player who moved to St Johnstone after a spell in England, midfielder Scott Brown, is aiming to push on after making his first start for the club in the 4-0 defeat by Motherwell on New Year's Day.
The 19-year-old played almost 70 minutes of the action-packed Fir Park encounter before going off with the score 2-0 to the hosts.
Brown, who signed for Saints in the summer following two years with Bradford, said: "Obviously it wasn't the result that I wanted but I was pleased to make my debut and hopefully I have done enough to remain in the gaffer's plans for the coming season.
"The first goal was vital in the game and from then on we were always trying to fight back. We could have had three or four but arguably they could have had three or more than they got.
"I'm desperate to play again. We have a good squad and centre-mid is a key position and we have a lot of good central midfielders.
"It was a really physical game but you just have to deal with it. It's men's football now, not under-20s. I felt I dealt with it alright and hopefully I will get more opportunities."