Can someone tell me how OOAD is different from RUP and SDLC(waterfall) methodologies?
Object-oriented analysis and design is a term that describes two processes related to creating object-oriented software. These are only two steps. There is a lot more.
The Rational Unified Process (RUP) is a hybrid version of the Unified Process (UP). The UP is an iterative software development process framework. It can be used for creating any type of software application.
Software Development Life-cycle is a generic term which describes the life-cycle of software development. Company A has a different SDLC than company B. Company X has a different SDLC than company B.
These three terms represent very different aspects which have common ties and use some common terminolgy.
For example, a RUP-based process may include OOAD as two of the steps. Or, company U which develops object-oriented software may include OOAD as part of its SDLC.