MSP30 launchpad: sending multiple char in UART, with each char must be sent only after acknowledgement

by the example , if char 'u' is received send "hello world".. we can write multiple if conditions if we will receive different characters and transmit accordingly.. but i have to transmit multiple characters and receive accordingly..

for example,

I have to send 'h', 'i' through UART.  first 'h' should be sent. if "OK" is received send "i".

how to write UCA0TX interrupt?