When Obito first awakened his Sharingan it already had two tomoe and he was able to take immediate advantage of it despite his inexperience. His eye gained three tomoe after witnessing Rin's death, moments before his Mangekyō Sharingan awakened. After giving up his original left eye to Kakashi, Obito obtained a new left Sharingan eye and has multiple Sharingan placed in storage should he need them. As an adult, Obito seems to be able to keep his Sharingan active at all times with minimal drain on his chakra levels.
He can use his Sharingan to perform various genjutsu such as forcing Konan to revealing the whereabouts of Nagato's body, and trapping both Fū and Torune in a genjutsu for a considerable amount of time.
Like Madara, Obito can use his Sharingan to place tailed beasts under his control,
the control more effective on a single target with some difficulty when controlling more than one at the same time, allowing the beasts to temporarily break free in such instances. Obito can also perform Izanagi,
a technique the Uchiha deemed a kinjutsu since the user's Sharingan is stricken with blindness and sealed away forever. While there are normally limits on how long Izanagi can remain active, Obito used Hashirama's DNA to extend the length of his Izanagi, allowing him to keep it going for five minutes while only losing one of his replacement left eyes.
The Sharingan grants Obito the use of a phasing ability best described as "intangibility". The eye serves as a gateway to another dimensional space, which he can instantly send parts of his body to and bring back from. This causes him to seem "intangible" in the original dimension; when struck, attacks pass right through him, though he sometimes pretends to be hurt or cry out in pain to toy with his opponents. Obito must become solid in order to interact with others, leaving him susceptible to counter-attack when he does so. He can only make himself continuously intangible for about five minutes. He can extend this intangibility to whatever he's touching at the time.
Overall Obito favors using his intangibility for defense, letting his opponents and their techniques phase through him and absorbing them while they are off-guard. This became such a key element in Obito's fighting style that he rarely resorts to more traditional forms of ninjutsu. The only reliable counter to this technique has been Kakashi's own Kamui, ironically used by the counterpart, left eye; objects warped away by Kamui are sent to the same dimension which can then harm the part of Obito that may exist in the other dimension at that time. Because of the ability to warp himself and the fact that the eyes share the same dimensional void, Obito is "immune" in a sense, to Kakashi's use of this technique on him.