I am inclined to disagree.
To take part "right from the start" Romania has to declare war somewhere around the austrian DoW against Russia (6th August 1914). To act quickly after DoW against Austria as IOTL for that Romania has to start mobilization at least end of July ... absolute hig-time of harvest (different to OTL DoW end of August 1916, end of August the harvest is in its last rows).
Somehow I doubt, that Romania would be able to fully mobilize their units completly ITTL, given the level of literacy and informational as well admistrative infrastructure in the region of its peasant-soldiers. Likely many won't even show up at their garrisons.
Let's assume - for the sake (?) of the romanians - that Conrad and the austrians completly ignore this build-uip of forces at their borders to Romania and continue their already f--ked up deployment as IOTL and midth to end of August - same imt as austrian 1st Aermy advances towards Krasnik - the worse than IOTL in 1916 equipped and manned romanian armies attack as IOTL into Hungary.
There tzhey will meet almost the same they met IOTL : some border guards, who quickly retreat. Due to their even worse logistics (more animals still on harvesting duties or "on the way" they won't get further as IOTL, where they stopped for ... logistical reasons.
Instead of being sent to reinforce austrian 3rd army and esp Army Group Kövess, austrian 2nd Army will be deployed to Transsylvania (most of it). For the galician front this will mean :
lesser holes plugged than IOTL in austrian 3rd Army
earlier loss of Lemberg (if russian 3rd Army aka Gen. Ruzsky gets his ass moved, something he isn't well known for)
Kövess, one of the few capable austrian commanders would with his lesser forces retreat towards the Carpathians earlier - to set up defensive perimeters.
Probably the "rest" will go mainly as IOTL in Galicia in 1914, with "stopping"the russians for the same reasons at the feet of the Carpathians as IOTL : logistics.
Meanwhile, the finally aseemble austrian 2nd Army, though still not at "standards" of the germans/french/russians but more than well above romanian standards two years earlier than IOTL will drive back the romanians at least to the border passes again.
Meanwhile Bulgaria will most likely become much ealier inclined to join the CP, as Romania, quite occupied in Transsylvania is an open invitation to grab the Dobruscha. Though they won't support in late 1914 the austrians in Serbia as IOTL 1915, they would invade Romania.
Romania will now - attacked on two sides - cry for russian help which will cause lesser russian troops in Galicia and ... Poland, increasing the chance of a CP-success at the battle at the Vistula with taking of Lodz, Ivangorod and maybe even Warsaw at the change from 1914 into 1915.
and there are several other ... opportunities how things would/could go.
A romanian entry in August 1914 is everything ... but nowhere near a war-winner - for the Entente.
There's also the small matter of the six Russian divisions that were guarding the Romanian frontier. With the Romanians as allies, they present some interesting opportunities. the logical use is to reinforce Romania and that eliminates any Austrian offensive against them.
As for Bulgaria joining the war earlier, that smacks of desperation. the Bulgarians are pretty cautious and take their sweet time. Romanian intervention makes the Bulgarians less likely to help as they wouldn't have the Turks to help them, the Austrians are quite busy and the Serbs and Russians still in the fight. Bulgaria is only fighting if they think they can win. they aren't sure of that in August 1914 OTL and they would be less likely ITTL.
So the most likely result:
The Austrians are unable to send B-Staffel to Poland. The Russians have a much larger advantage and punish Conrad for his stupidity. Meanwhile, the Romanians aided by the six Russian divisions and the Serbians make a real Southern Front for the Austrians.
Politically, the Bulgarians and the Ottomans hesitate. With the Russians are real force in the Balkans, the Bulgarians know they might go down hard. The Ottomans will fear a possible Entente Bulgaria. Given the huge gains that Romania and Serbia are expecting, they might be willing to bribe the Bulgarians with the territory they lost in the Second Balkan War