What is the nature of the oxide of boron? I think it should be acid, as I am aware that a compound by the name of Boric acid exists. But how can the oxide be acidic when it is electropositive? Also, other members of its group are well known metals. Why isn't the oxide amphoteric, when its group member Aluminium has an amphoteric oxide? 

My teacher told us that the oxide of boron is basic in nature? So what exactly is its nature?
The most common oxide of boron is boron trioxide, $\ce{B2O3}$, which is the anhydride of boric acid:

$$\ce{H3BO3 -&gt; HBO2 + H2O}$$
$$\ce{2HBO2 -&gt; B2O3 + H2O}$$
$$\text{net} \ \ce{2H3BO3 -&gt; B2O3 + 3H2O}$$

As the anhydride of an acid, boron trioxide is acidic. It reacts with water to regenerate the acid:

$$\ce{B2O3 + 3H2O -&gt; 2H3BO3}$$

Boric acid's acidity does not derive from the dissociation of a proton (or abstraction of a proton by water), as does many acids, for example sulfuric acid:

$$\ce{H2SO4 &lt;=&gt; H+(aq) +HSO4- (aq)}$$
$$\ce{H2SO4 + H2P &lt;=&gt; H+(aq) + HSO4- (aq)}$$

Boric acid (written now as $\ce{B(OH)3}$, which is suggestive of its structure) instead reacts with water by forming a dative complex which is the active acid. A water molecule associates with the empty $p$ orbital on boron. This difference in behavior is enabled also by the electropositive character of boron. The acidic oxides of some metals behave similarly. 

$$\ce{H2O + B(OH)3 &lt;=&gt;H2OB(OH)3 &lt;=&gt;H+(aq) + B(OH)4^- (aq)}$$